Document fail-closed rejection of empty permission route segments

[smiggleworth]

Documents the existing permission parser contract: empty route path segments invalidate a grant, and one invalid grant in the selected permission source rejects that whole claim. Authentication does not keep the other grants or fall back to a lower-priority source. Unused lower-priority sources are not parsed.

Client/Auth0 guidance describes refused CONNECT authentication; MCP OAuth guidance describes invalid fitz_permissions provisioning as 403 insufficient_scope. Explicit malformed path examples avoid confusion with the scheme separator.

A regression mixes valid grants with kv://#read and a valid lower-priority scope, then requires normalization to fail. There are no production code changes.

Validation

• Preparation passed 38 claims and 5 OAuth tests.
• RED: bypassing the parser's empty-segment rejection made the whole-claim regression accept the malformed grant; restoring it passed.
• Preparation format, diff and full locked/all-target/all-feature pedantic Clippy passed; compiled worktree paths and the new regression were audited.
• Independent preparation review found the docs consistent with source and selected-source behavior.
• Final candidate bb7491a is based on merged develop c241d45. Its four-file +47/-2 patch and blobs match reviewed preparation; merged UI is preserved, with no unmerged Refresh dirty admin projections before bounded admin snapshots #464 changes.
• Fresh 38 claims and 5 OAuth tests passed. Full locked workspace passed 2,835 tests across 49 parent targets (7 ignored; unit 2,151 passed, 3 ignored). Format, diff and full locked/all-target/all-feature pedantic Clippy passed.
• Independent final review is clean; MCP malformed-grant HTTP behavior is source-verified, without claiming a new runtime test.
• Fresh exact-head CI passed backend, frontend and both native AMD64/ARM64 container jobs. CodeQL passed Rust, JavaScript/TypeScript and Actions analyses.
• Stream run 38159000765 attempt 1 passed both complete 32-workload default-sampling campaigns, all eight original budgets in each campaign and all four unchanged controls. Independent timing recomputation and artifact provenance audit are clean. All three benchmark executables are byte-identical across phases; both full source-mode timing campaigns still ran (228 captures).
• Immutable Stream artifact 11710574729 ZIP SHA256: 29025f8492696a131f5205902f09bda4a2e77f062c94d8fa2b81edf0a1732ac1, matching the GitHub digest. The audit checks fresh recorded Rust inputs/build artifacts and the original budgets; three embedded Markdown resources are Git-blob verified, without claiming workflow-recorded build-time hashes.

AI assistance: Codex clarified documentation and independently reviewed existing behavior.

Fixes #430

[smiggleworth]

Adversarial source review of 9f40c8b. Reviewed fail-closed permission normalization and source precedence; docs match authentication path. Integration with the latest develop and fresh hosted checks remain required before merge.