perf: move loopback load origin to Rust

[seonghobae]

Fixes #34.

Verified complete succession — closed without merge credit

This historical Draft proved that Python ThreadingHTTPServer inside k6 http_req_duration polluted gateway attribution. Its valid delta was bounded to four paths: .github/workflows/ci.yml, TEST_STRATEGY.md, tests/load/load_origin.rs, and removal of tests/load/upstream_fixture.py. It introduced a std-only Rust HTTP/1.1 measured origin and used it for both generic and routed pg-erd load without weakening 4 VUs / 400 iterations / zero HTTP failures / p95 <20 ms acceptance.

The delta is now completely succeeded by the ordinary current stack, so this PR is closed rather than restacked or merged:

• final fix: keep Pingora diagnostics payload-free #31 exact 8230f8d1311adccc133b7f6ed86df441533e486c contains no tests/load/upstream_fixture.py and its load job contains no Python invocation;
• current tests/load/load_origin.rs is a strict superset of the historical fixture: still std-only Rust with the 64 KiB header bound, but now with bounded workers/queue, startup validation, connection-mode/service-delay controls, direct rustc tests and constraint-oriented documentation;
• current generic load still executes 4 VUs / 400 iterations with exact status/body, zero failures and p95 <20 ms;
• current pg-erd load still executes 4 VUs / 400 iterations through the compiled cwl-pingora-pg-erd-migration path, and strengthens the historical contract with independent backend/frontend p95 <20 ms plus at least 198 samples per route;
• final fix: keep Pingora diagnostics payload-free #31 CI 34251472731 is terminal success and published exact-SHA generic and pg-erd k6 artifacts (k6-loopback-8230f8d... and k6-pg-erd-loopback-8230f8d...). Supply Chain 34251472751 is also terminal success.

Historical #35 source/test/evidence is therefore not being discarded: every still-valid generic and pg-erd fixture/contract has been integrated or strengthened and re-executed on a verified successor. Historical capacity measurements on old descendants remain historical evidence only and are not promoted into production SLO credit.

This closure does not imply #33 or later descendants are GREEN; every changed descendant must independently reacquire exact-head execution/review. It also does not grant protected merge, immutable release, canary, cutover, rollback or legacy-removal credit.

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