fix: skip unchanged L0 retention checks

[UniversePeak]

Summary

Avoid repeated replica LIST requests from the L0 retention monitor when a database's local L0 watermark and cached L1 watermark have not changed. The retention pass now serializes per database, reuses the existing L1 max-file cache, and retries time-only eligibility changes at the next eligible timestamp.

Problem

Store.monitorL0Retention runs every L0RetentionCheckInterval (15 seconds by default) and DB.EnforceL0RetentionByTime previously listed the replica at level 1 and level 0 on every tick, even when an idle database had not changed. That creates up to 5,760 unnecessary LIST requests per database per day.

Solution

• Track the local L0 max TXID and last successful L1 watermark in DB state.
• Skip the remote retention scan when those inputs are unchanged and no file is waiting to become old enough.
• Reuse MaxLTXFileInfo for the cached L1 max file instead of listing level 1 directly on every pass.
• Reset the retention state when local state or the configured L0 retention window changes.
• Add a regression test that counts level LIST calls across unchanged and new-L0-file passes.

Scope

In scope:

• Avoiding redundant L0 retention LIST operations for unchanged databases.
• Preserving retention checks when a new L0 file appears or a time threshold is reached.

Not in scope:

• Changing retention policy, intervals, or replica layouts.
• Altering compaction or replication behavior outside the retention check.

Test Plan

• CGO_ENABLED=0 go.exe test -run TestDB_EnforceL0RetentionByTime_SkipsUnchangedDatabase -count=10 . (pass)
• CGO_ENABLED=0 go.exe test -run '^$' ./... (pass; all packages compile)
• CGO_ENABLED=0 go.exe vet ./... (pass)
• go test -race ... was attempted but this Windows host has no CGO compiler/toolchain; Go reported -race requires cgo.
• pre-commit run --files db.go db_test.go store.go was attempted; hook environment setup was blocked by a network read timeout downloading setuptools. gofmt and git diff --check pass.

The full package test suite was also attempted, but this Windows environment's file-backend tests fail with Access is denied while syncing temporary directories; this is independent of the focused regression test, which passes repeatedly.

Related

• Fixes L0 retention monitor LISTs the replica for every database on every tick, with no change-detection guard #1468