Per-export cooldown compaction gating

[jaredLunde]

What

Adds a per-export, default-OFF cooldown knob that defers dead-ratio compaction of a chunk until it has been idle (unwritten) for compaction_cooldown flush cycles.

On overwrite-heavy DB volumes a hot chunk repeatedly crosses the dead-ratio threshold and gets recompacted over and over, rewriting its live blocks to S3 each time — pure PUT-byte write amplification. Trace-replay (compaction_sim) showed a simple cooldown age-gate cuts S3 PUT bytes ~22% (Postgres) / ~30% (SQLite) at 128 KiB blocks, ~0 elsewhere — and that the fuller F2FS (1-u)/(2u)·age victim score converges to the same result, so we ship the simpler gate.

How

• New compaction_cooldown: Option<u64> (0/unset = disabled) on ExportConfig + the PUT /api/exports/{name} DTO. Persists via the existing export.json save/discover and is preserved across resize_export.
• compact_if_needed gains cooldown + a per-chunk idle-age map; gates only the dead-ratio candidate branch (also skips its pack-index fetches for hot chunks). The pack-count cap (> 16 packs) stays an unconditional greedy backstop, so a hot chunk's packs can never grow without bound.
• The flush scheduler owns an ephemeral per-chunk idle-age map, advanced each cycle from the new FlushStats.touched_chunks (increment all, reset written chunks to 0, prune singletons). Ephemeral by design: a daemon restart just defers reclaim briefly (bounded by the pack cap) — never an incorrect compaction.
• Resolves the write-trace hook TODO in create_export (GLIDEFS_WRITE_TRACE_DIR) — the production counterpart that makes this analysis reproducible on real traffic.

Safety

Default OFF is byte-for-byte the current code path. A buggy age map can only defer compaction, bounded by the pack cap — never a durability or correctness issue.

Tests

Integration tests (in-tree harness, in-memory object store, cold-reader data-integrity verification):

• test_cooldown_compaction_defers_hot_chunk — unknown/age-7 defer, age-8 compacts; latest data survives.
• test_cooldown_pack_cap_overrides_hot_chunk — pack cap fires on a hot chunk regardless of cooldown.
• test_flush_reports_touched_chunks — FlushStats.touched_chunks correctness.
• test_compaction_cooldown_persists_across_discover — save→discover round-trip.

cargo build + cargo clippy clean; existing compaction/GC/flush tests pass.

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