perf(sessions): skip unchanged host bookkeeping writes on idle passes

[devin-ai-integration]

Summary

• An idle history pass no longer commits changes to the session stores. Before this change, each pass rewrote every provider's host-coverage://<provider>/v1 row and the Kimi and Pi discovery frontiers, even when nothing had changed. Each idle project pass made 9 WAL commits, plus about the same number on user-sessions.db. The daemon runs this pass every minute according to HISTORY_IDLE_RECHECK_INTERVAL.
• The new revise_host_record in runtime/source.rs writes a record only when its byte_offset or file_id changes. Readers check only these fields. A real change still updates mtime, allowing a record to move byte_offset backward past the monotonic cursor guard.

Motivation

Refs #2521, Refs #2658.

I measured the path again after #2777 and #2948. The capture path already reads only the objects named by a commit. In load_current_mutations, only sources with complete snapshot absence reach SourceObjectCoverageV1::Complete.

I then inspected an idle daemon with no active ingestion, using an isolated profile and project. Temporary SQL probes, which have since been removed, and parse_offsets snapshots showed which rows changed each minute. Both stores rewrote host-coverage://{claude,cursor,kimi,opencode,pi}/v1 and host-frontier://{kimi,pi}/discovery/v1, even though only mtime increased. Every commit adds WAL frames that readers and checkpoints continue to reread while the daemon is idle. #2658 identifies this idle traffic as the reason that whole-daemon bytes per message cannot be measured.

Changes

• crates/tracedecay-sessions/src/runtime/source.rs: persist_host_provider_coverage now uses the new revise_host_record, which skips unchanged records.
• hosts/kimi.rs and hosts/pi.rs: discovery frontier writes now use revise_host_record. Reaching the end on a frontier that is already reset no longer writes.
• ingest/project_provider.rs and ingest/user_provider.rs: remove the Codex callers' stored_coverage != Some(coverage) guard. The shared write now performs this check for every provider.
• crates/tracedecay-session-runtime/tests/session_store_read_cost.rs: adds idle_history_pass_commits_nothing. The test runs the production project ingest pass twice over an empty host home on a registered store. It verifies that the first pass commits and that the second appends no WAL commit frames.

Test plan

• bash scripts/require-exact-test.sh cargo test -p tracedecay-session-runtime --features test-helpers --test session_store_read_cost -- idle_history_pass_commits_nothing --exact: 1 passed. Without the tracedecay-sessions change, it fails with left: Some(9) right: Some(0).
• cargo test -p tracedecay-sessions --lib -- runtime::hosts runtime::ingest runtime::source: 353 passed.
• cargo clippy -p tracedecay-sessions -p tracedecay-session-runtime --all-targets --features tracedecay-session-runtime/test-helpers -- -D warnings: clean. cargo fmt --all -- --check: clean.
• Tested manually on a real daemon built from this branch with an isolated HOME/TRACEDECAY_DATA_DIR and an enrolled project. Before the change, the daemon rewrote coverage and frontier rows every idle minute. After the change, parse_offsets and both stores' WAL mxFrame remained byte-identical for 130 s, covering two idle passes.

Not covered: this PR does not remeasure operator-scale read volume, reported as 2 TB in 3 h in #2521. It does not change the per-message store-growth target in #2658.

Checklist

• CHANGELOG.md: 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/868821e98d0743dc83648344702f3479
Open in Devin Desktop: https://app.devin.ai/desktop/session/868821e98d0743dc83648344702f3479?variant=devin
Requested by: @ScriptedAlchemy

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