fix(index): seal an edit's successor during text projection

[devin-ai-integration]

Summary

• If an edit arrives while a publication's text projection is running, the worker now seals the successor generation during the projection join. Before this change, the edit waited for the projection to finish and its generation to be seated.

Motivation

Fixes #2798. The publishing pass called .await on published_text_projection directly, so the worker could not act on a pending arrival until the projection finished and the seat completed.

Changes

• registry/mount.rs: the projection join now loops with tokio::select! on the projection and worker_wake.
  • While the projection runs, a pending arrival triggers a successor seal. The pass only calls try_acquire_owned() on background_reconcile_admission and never waits on it, because it holds the publication gate.
  • The seal uses the same branch order as the source pass: first reconcile_retained_text_generation_with, then activate_or_reconcile (graph-on) or reconcile_now (graph-off).
  • A Published outcome is stashed in sealed_successor. The next pass returns it instead of resealing from source, and the arrival stays pending for that pass.
  • The post-projection source re-check is skipped when a successor is already sealed, so it does not race that seal.
  • The stash is cleared when publication authority becomes terminal.
• New test-only gate pause_next_opened_published_text_projection (registry.rs, test_gates.rs). It holds a publication's projection after its first advance opens the build, so the publishing pass stays in the join.
• New regression test edit_during_text_projection_seals_its_successor_before_the_projection_finishes (tests/reconcile.rs). It edits src/lib.rs while the projection is held and requires the durable pointer to move before the projection is released. It then checks that both generations project and that the served successor has the edited symbol.

Test plan

• New test fails before the fix (10 s timeout waiting for the pointer to move) and passes after: bash scripts/require-exact-test.sh cargo test -p tracedecay-code-index-runtime --lib code_index_scheduler::tests::reconcile::edit_during_text_projection_seals_its_successor_before_the_projection_finishes -- --exact
• cargo test -p tracedecay-code-index-runtime --lib: 587 passed, 1 ignored
• cargo clippy -p tracedecay-code-index-runtime --all-targets -- -D warnings is clean
• cargo fmt --all -- --check, python3 scripts/linux-test-partitions.py check, node scripts/lint-commit-range.mjs --repository . origin/master HEAD pass
• cargo nextest run --workspace --no-fail-fast (not run locally; left to CI)

Checklist

• CHANGELOG.md updated (not updated; release-please generates it)
• No secrets, credentials, or .env files included
• No breaking changes

Link to Devin session: https://app.devin.ai/sessions/4f29111e045a46a3a79417028ae66e8c
Open in Devin Desktop: https://app.devin.ai/desktop/session/4f29111e045a46a3a79417028ae66e8c?variant=devin
Requested by: @ScriptedAlchemy

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[devin-ai-integration]

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