feat(backend): return fenced subscription activation receipts

[sandersonstabo]

Depends on #326.

Scope:

• add an opt-in request-handler-owned conversation subscription registrar
• return non-Clone local activation receipts while leaving successful subscriptions Pending
• activate only through an explicit post-response method and stop subscriptions immediately/idempotently
• fence receipts to their exact handler so cross-handler lease collisions cannot activate
• preserve ordinary request behavior and add real-repository coverage for fresh/resume/error/replacement/stop/delegation paths

Excludes:

• connection or transport integration
• delivery tasks, patch publication, notifications, or QUIC ownership
• shared build/manifest changes

Verification:

• external Luna worker focused suite: 40 passed
• authoritative jobs=1 Bazel gate: //modules/backend:backend and //tests/backend:request_handler_test
• fresh production/test Clippy markers
• root //:format_test
• locked/offline Cargo metadata, exact seven-path hash/status guards, and Bazel shutdown
• receipt: D:/artisan-editor-gate-logs/backend-request-handler-subscription-receipt-20260829-gate1/receipt.json

Remaining work:

• the next packet must adapt the connection driver to transport's accepted receipt-aware dispatch and call activation only after response write plus send-side FIN
• conversation delivery ownership remains separate

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Summary by cubic

Adds an opt-in request-handler-owned subscription registrar that returns fenced, non-clone activation receipts. Subscriptions are prepared as Pending and left for the caller to activate after writing the response; ordinary respond behavior is unchanged.

Behavior

• Activation happens only through RequestHandler::activate_after_response, which returns the exact lease and cursor or a typed error.
• Receipts are bound to the exact handler that prepared them, so cross-handler activation is rejected.
• Unsubscribe stops immediately and idempotently, without producing a receipt.
• Subscription preparation failures reuse the existing wire classifications.

Tests

• Adds real-repository coverage for fresh, resume, beyond-tail, repository-failure, replacement, cross-handler, stop, and delegation paths.

^{Written for commit b974e24. Summary will update on new commits.}