fix(code-index): stop reading a held projection slot as work

[devin-ai-integration]

Summary

• text_projection_needs_work reported any contended slot lock as work in progress. Another probe, or a no-op advance over ready owners, holds that lock too. When the two collided, the published text projection stamped a phantom worker continuation, and freshness stayed Verifying / Pending at the serving swap. This is the reconcile flake in test: four intermittent master failures (reachability, reconcile, git_watch, dashboard) #3041.
• Fixes the git-watch linked-worktree debounce flake, which was a test-clock race.

Fixes #3041

Motivation

#3041 lists four intermittent master failures. Status of each:

• Reconcile freshness, (Some(Verifying), Pending): failed 59 of 3,000 runs under local stress. I instrumented every pending-wake writer. There were two late wake sources:
  1. The test's request_complete_generation demand sometimes landed after the first pass had claimed its wake. That queues a real follow-up pass, so Verifying is the correct reading. Fixed in the test by holding admission until the demand is posted.
  2. Every remaining failure was the published projection task's note_worker_continuation, fired because the slot try_lock returned WouldBlock while the slot was idle. This is a production bug, fixed here.
• Git-watch debounce (tests.rs:643): the test ran on start_paused time. The paused clock auto-advanced past the test's deadlines while a live notify wake was still in flight. The watcher's own copies of the marker writes could also produce a second drain.
• Reachability: PR fix(code-index): serve a complete seat that lands after the read misses #3044 already addresses this. With fix(code-index): serve a complete seat that lands after the read misses #3044 and this branch, the test passed all 80 stress runs. The issue reported 5 failures in 40 runs on the earlier master.
• Dashboard ChunkLoadError: that message is the error RouteChunkBoundary.dom.test.tsx deliberately rejects with. It appears in the output whenever that suite runs, so it does not identify the failing test. The suite passed 15 of 15 local runs. No change here.

Changes

• code_index_scheduler/serving.rs: CodeTextProjectionStateV1 keeps the slot's idle state in an atomic (slot_idle), written on every slot transition through replace_slot. The probe reads that atomic and never takes the lock, so it still never queues behind an advance's slice.
• tests/serving.rs: adds a_held_idle_projection_slot_is_not_text_projection_work. It holds an idle slot's lock and asserts no work is reported. It fails before the fix (serving.rs:908) and passes after.
• tests/reconcile.rs: ready_wait_ends_only_after_the_graph_tail_seats_the_generation holds admission until the complete-generation demand is posted.
• git_watch/tests.rs, state.rs, git_watch.rs: the linked-worktree test now runs on real time. It waits on a test-only operation_held signal (debounce observed InFlight) and asserts the release with >= 1 drain instead of an exact count.

Test plan

• bash scripts/require-exact-test.sh cargo test -p tracedecay-code-index-runtime --lib <test> -- --exact passes for each of the three tests above, after the rebase on origin/master
• Stress results:
  • reconcile ready-wait: 0 failures in 3,000 runs (59 before)
  • git-watch linked hold: 0 failures in 2,000 runs
  • reachability: 0 failures in 80 runs
• cargo test -p tracedecay-code-index-runtime --lib -- code_index_scheduler::tests::serving code_index_scheduler::tests::reconcile git_watch: 280 passed
• cargo fmt --all -- --check and cargo clippy -p tracedecay-code-index-runtime --all-targets -- -D warnings are clean
• python3 scripts/linux-test-partitions.py check and node scripts/lint-commit-range.mjs --repository . origin/master HEAD pass
• Full workspace cargo nextest run: left to CI

Checklist

• CHANGELOG.md updated (not updated; release-please generates it from commits)
• No secrets, credentials, or .env files included
• Breaking changes documented (none)

Link to Devin session: https://app.devin.ai/sessions/e8f223a3ec5c476885a5d8903bafa4c2
Open in Devin Desktop: https://app.devin.ai/desktop/session/e8f223a3ec5c476885a5d8903bafa4c2?variant=devin
Requested by: @ScriptedAlchemy

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[devin-ai-integration]

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• Disable automatic comment, CI, and merge conflict monitoring

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[devin-ai-integration]

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