[WRONG BRANCH] fix(responses): scope compact handoff cache to authenticated admission principal

[luvs01]

Motivation

• Prevent cross-principal rerouting where a process-global compact-handoff cache keyed only by caller-supplied lane headers could cause one authenticated client to influence another client's compact request routing.
• Ensure fallback reroutes only apply within the same authenticated admission principal and do not leak a victim's compact body or quota to an attacker-visible provider.

Description

• Add compactHandoffRouteKey(req, admission) that composes the resolved DataPlaneAdmission principal with the session lane to key handoff state, and treat loopback/admission-less requests as ineligible.
• Replace the previous process-global lane-only helpers with rememberCompactHandoffRoute, forgetCompactHandoffRoute, and compactHandoffRoute that accept the admission and use the namespaced key.
• Wire the new admission-scoped key into handleResponsesCompact so remembered routes are stored, retrieved, and cleared only for the same authenticated principal.
• Update the focused regression test (tests/responses-compaction-routing.test.ts) to assert same-principal handoff still works while a different configured principal and a loopback caller cannot consume the cached route.

Testing

• Ran bun test tests/responses-compaction-routing.test.ts which passed (focused compaction-routing suite).
• Ran bun run typecheck which passed.
• Ran bun run privacy:scan which passed.
• Executed the full test run during validation; the focused compaction tests passed and the change did not affect the compact routing behaviors, while one unrelated full-suite run previously observed a timeout in a different test (environment-dependent) and is not related to this change.

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Codex Task

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Codex Review Summary

This comment shows the latest Codex review activity on this pull request.

Review	Status	Commit	Review trigger
📝 Code Review	✅ Completed 2026-08-31T00:59:38.302715Z	036ef81	PR opened

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[github-actions]

⏳ DRAFT

• wrong target branch (main); retarget to dev.

What to do

• Retarget this PR to dev — all contributions go to dev.

Its title has been prefixed with [WRONG BRANCH].
This pull request was already a draft. Its draft status will be preserved after every issue above is resolved.