fix(graph-db): keep a serving code-shard head through the replay sweep

[devin-ai-integration]

Summary

• During the superseded-generation replay sweep, retain every relational verified head in a code-shard: namespace. A code-shard head remains the serving authority until a successor publish supersedes it, whether or not an in-memory lease is installed. The sweep therefore returns Retained and never reaches retire_verified_head_replay.
• Add a contract test for both mid-publish paths: one with an installed lease and one after a remount without a lease. The test also checks that the ordinary path retires the head after the successor publishes.

Motivation

Fixes #1226. After a SIGTERM mid-refresh, the next daemon restart replayed the entire sealed generation instead of resuming publication.

publish_atomically moves the durable pointer at seal, but the graph head CAS lands later. During the text-artifact build, the previous generation's replay remains the projection's relational verified head. If maintenance deletes that superseded generation's file and runs the release reconcile during this gap, the sweep treats the shared code-shard head as disposable. The per-generation retention logic excludes candidate heads from retained and routes the selection through retire_verified_head_replay, which deletes the head row.

The restart then finds verified_head = None and logs code_index_graph_head_recovery_degraded with "replay the exact sealed generation to repair its quarantined graph projection". It re-climbs every file. The race between the head CAS and the maintenance tick caused the flaky pass rate.

Under the canonical code-shard: namespace, the code must never retire a head that is still installed. The head is the serving projection and may move only through a successor CAS. After that CAS, the release retries and reclaims the old replay through the ordinary path. The per-generation head-retirement exemption remains unchanged for the legacy layout. In that layout, each generation is the permanent head of a one-member projection, and head retirement is the drain mechanism.

Changes

• crates/tracedecay-graph-db/src/registry/publication.rs: the heads census in retire_one_code_generation_replay retains code-shard head locators even when they match a deletion candidate, regardless of installed-lease state.
• crates/tracedecay-graph-db/tests/graph_db_suite/verified_generation_contract/code_graph_layout.rs: sweep_keeps_the_shared_head_while_the_deleted_generation_still_serves asserts Retained and preserves the head while the deleted generation still serves. It covers both an installed lease and a remount without one, then checks for Retired through the ordinary path after the successor publishes.

Test plan

• cargo nextest run --workspace --no-fail-fast passes (ran cargo test -p tracedecay-graph-db --features test-helpers --test graph_db_suite: 157 passed, 0 failed)
• cargo clippy has no new warnings (cargo clippy -p tracedecay-graph-db --all-targets --features test-helpers -- -D warnings clean)
• New regression test verified red on master (returns Ok(Retired), head deleted, including the no-lease remount path) and green on this branch; run via bash scripts/require-exact-test.sh cargo test -p tracedecay-graph-db --features test-helpers --test graph_db_suite verified_generation_contract::code_graph_layout::sweep_keeps_the_shared_head_while_the_deleted_generation_still_serves -- --exact
• cargo fmt --all -- --check clean; python3 scripts/linux-test-partitions.py check clean (no new test target)

Checklist

• CHANGELOG.md updated (under [Unreleased] if no version bump)
• No secrets, credentials, or .env files included
• Breaking changes documented (if any)

Link to Devin session: https://app.devin.ai/sessions/3c49720ba0f64d208d7aae99d4851be6
Open in Devin Desktop: https://app.devin.ai/desktop/session/3c49720ba0f64d208d7aae99d4851be6?variant=devin
Requested by: @ScriptedAlchemy

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[devin-ai-integration]

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