docs(spec): reframe 7-segment key format as SDK convention; add server-side requirements

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Companion to cachekit-io/saas#231 (closes cachekit-io/saas#91).

The CachekitIO validator is now security-only; the 7-segment grammar is Python SDK convention, not a server contract.

• cache-key-format.md: new Server-Side Requirements section documenting exactly what the server enforces (percent-decoded transport, 400-char cap, [a-zA-Z0-9_.:-] charset, .. rejection, ns:/nsapi: namespace shape, write-space split), plus a note on the Full Key Structure marking it SDK-internal.
• interop-mode.md: status DRAFT → SERVER-SUPPORTED — the validator accepts {ns}:{op}:{hash} keys (scoped to the default namespace); SDK-side generation remains pending (design: cross-SDK interop mode for cache key and value format #1).
• test-vectors/cache-keys.json: key_format/note fields updated with the convention-vs-contract framing. No vector values changed — cross-SDK byte compatibility untouched.

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Conflict with main in two files, both substantive content disagreements rather than non-overlapping additions:

• spec/interop-mode.md: HEAD's status line says "SERVER-SUPPORTED... SDK-side generation not yet implemented"; main's says "SPECIFIED (interop/v1)... NOT yet implemented in any SDK." Different factual claims about the same status — picking one drops the other.
• test-vectors/cache-keys.json: differing generator version, ci_verification field, note, and key_format text between branches — same fields edited with different content on each side.

Neither is a safe automatic union; needs a human call.

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Auto-rebase attempted: merging origin/main produced conflicts in spec/interop-mode.md (contradictory status claims — this PR says "SERVER-SUPPORTED", main says "SPECIFIED, NOT yet implemented") and test-vectors/cache-keys.json. Picking a side changes the spec's meaning — needs a human call. Left the branch untouched.

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Auto-rebase attempted a merge of main but hit conflicts in spec/interop-mode.md and test-vectors/cache-keys.json. The doc conflict is a direct contradiction, not a union — this branch says interop-mode status is "SERVER-SUPPORTED" while main says "SPECIFIED, NOT yet implemented in any SDK." Picking one over the other is a factual call I can't make safely, so I left it for a human.

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Auto-rebase sweep: merging main conflicts on two files with genuinely contradictory content, not additive changes:

• spec/interop-mode.md: this branch marks interop-mode as "SERVER-SUPPORTED"; main marks it "SPECIFIED (interop/v1) — normative, NOT yet implemented" with different linked references. Picking either overwrites a real status claim from the other side.
• test-vectors/cache-keys.json: this branch vs main disagree on generator version (v0.5.0 vs v0.12.0), key_format description, and whether a ci_verification field exists.

Needs a human call on which status/version is current. Left unresolved.

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Merge conflict against main in two files, both genuine content disagreements rather than additive unions:

• spec/interop-mode.md: this branch claims Status is SERVER-SUPPORTED (SDK-side generation not yet implemented), main claims SPECIFIED/normative and NOT yet implemented in any SDK — contradictory implementation-status claims.
• test-vectors/cache-keys.json: differing generator version (v0.5.0 vs v0.12.0) and differing note/key_format metadata for the same vector set.

Picking a side asserts one version's facts over the other without verifying against the actual cachekit-py implementation — needs a human call.

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Auto-rebase sweep: merging main into this branch conflicts in spec/interop-mode.md (contradictory Status callouts — one claims SERVER-SUPPORTED with SDK generation pending, the other claims SPECIFIED/not-yet-implemented) and test-vectors/cache-keys.json (differing generator version, key_format, and note text, plus a ci_verification field only on one side). Both are substantive content conflicts, not a non-overlapping union — needs a human call on which claims are current.

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Auto-rebase check: base main has moved (main HEAD 936f22f) and this branch now conflicts in spec/interop-mode.md (contradictory status lines — "SERVER-SUPPORTED" here vs "SPECIFIED, not yet implemented" on main) and test-vectors/cache-keys.json (differing generator/ci_verification/note/key_format metadata). Both are genuine content disagreements, not independent additions — needs a human call, not safe to auto-resolve.

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Auto-rebase attempted, left for a human: merging main conflicts in spec/interop-mode.md (competing "Status" blurbs — SERVER-SUPPORTED w/ server-side-requirements link vs. SPECIFIED/normative-not-implemented) and test-vectors/cache-keys.json (different generator versions and note/key_format text). Both are real content conflicts, not a non-overlapping union — needs a call on which description is current.

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Auto-rebase attempted a merge from main but hit a genuine content conflict in spec/interop-mode.md: this branch's status line says interop SDK-side generation is "not yet implemented", while main now says it's "shipped opt-in in all three SDKs" — contradictory claims, not a non-overlapping union. test-vectors/cache-keys.json also conflicts. Needs a human call on which status text is current. Left branch untouched (no push).

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Auto-rebase: could not safely resolve merge conflicts against main (base @ 44ee4370f0604f2336833ac57d843d9747f9a691).

Conflicting paths:

• spec/interop-mode.md — the branches disagree on the actual status of Interop Mode: this PR's version still says SDK-side generation is "not yet implemented," while main now documents it as "SPECIFIED (interop/v1) — normative, and shipped opt-in in all three SDKs." Resolving this means asserting which claim is currently true, not a mechanical merge — needs a human call.
• test-vectors/cache-keys.json — overlapping edit tied to the same status change, same reasoning applies.

Leaving the branch untouched; no push made.

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Attempted auto-rebase onto main — conflicts in spec/interop-mode.md and test-vectors/cache-keys.json. Both are competing rewrites of the same status/note text: this branch reframes the interop status as "SERVER-SUPPORTED, SDK-side not yet implemented" and pins generator: cachekit-py v0.5.0, while main has since advanced to "SPECIFIED, shipped opt-in in all three SDKs" with generator: v0.12.0 and a new ci_verification field. Picking either side drops real content from the other — needs a human call on how to reconcile.

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Merge conflict against main in spec/interop-mode.md and test-vectors/cache-keys.json. main has since rewritten the status block (SERVER-SUPPORTED → SPECIFIED/normative, shipped opt-in across all three SDKs) — resolving means picking one side's status claim over the other's, not a non-overlapping union. Needs a human call on which status text is current.

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Auto-rebase sweep: merging main still conflicts in spec/interop-mode.md and test-vectors/cache-keys.json — both remain genuine content disagreements, not a non-overlapping union. main now asserts interop-mode is shipped opt-in in all three SDKs (Python 0.14.0+, TS 0.1.3+, Rust 0.4.0+), while this branch still claims SDK-side generation is "not yet implemented." The test vectors disagree on generator version (v0.5.0 vs v0.12.0) and main adds a ci_verification field this branch lacks. Picking either side asserts a factual claim about SDK implementation status I can't verify — needs a human call. Left branch untouched, no push.

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Auto-rebase check: merging main conflicts on spec/interop-mode.md and test-vectors/cache-keys.json. Both are substantive, contradictory edits (this branch's status blurb says SDK-side interop generation "is not yet implemented"; main's says it's "shipped opt-in in all three SDKs") — not a non-overlapping union, needs a human call on which status is current.

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Flagging for manual resolution. Conflicting paths: spec/interop-mode.md, test-vectors/cache-keys.json. The interop-mode.md status block is a factual contradiction, not a textual one: this PR asserts the CachekitIO validator accepts interop-format keys (security-only validation), while main's rewritten interop/v1 spec (#16) warns the deployed validator rejects them pending saas#91 — which closed hours before that warning was written, so which claim reflects the deployed validator is unverified. Even the cleanly-merging spec/cache-key-format.md hunks would contradict main's interop-mode.md warning. This PR likely needs re-authoring against the interop/v1 spec rather than a mechanical merge; resolving either way risks publishing wrong normative claims, so leaving the branch untouched.