fix(code-index): serve a complete seat that lands after the read misses

[ScriptedAlchemy]

Summary

code_facets sometimes answered Unavailable on a project whose index was ready. That made every_cursor_carrying_code_operation_mints_and_spends_a_continuation flake at application_production_reachability.rs:447.

Facets, timeline and source metadata resolve their generation through latest_complete_fresh_for_scope_awaiting_seat (queries.rs:357-360). After a publication, only the text owner is seated. The first complete read misses, and that miss posts the wake that decodes and seats the complete generation. The probe then read the serving_generation slot to classify the miss. If the decode won that window, the probe saw a seat and parked SeatNotServable, even though the seat had landed after the read and would have served it. #2823 dropped the re-check that had guarded this window.

Fix

The probe now snapshots the slots before the read. It parks SeatNotServable only when a seat was already present before the read declined it and is still present. A seat that lands after the miss returns None, and the seat's change signal re-probes and serves it.

Tests

The test(...) commit adds a #[cfg(test)] gate that holds the probe right after its miss. The regression test residency::a_complete_read_serves_the_seat_its_demand_lands_after_its_miss seats the decode inside that window. It fails on the test commit and passes on the fix commit.

• Before the fix, the acceptance test passed 30 of 32 hauler runs. Both failures were the reported code_facets ... terminate Unavailable panic.
• After the fix, it passed 40 of 40.
• cargo fmt --check is clean.
• cargo clippy -p tracedecay-code-index-runtime --all-targets -- -D warnings is clean.

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[devin-ai-integration]

✅ Devin Review: No Issues Found

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