perf(storer): remove per-chunk index lookups - #5615

…ion hints

Ensure GetIntoLoc defaults to fail-safe behavior by falling back to standard
verified GetInto whenever no sampling session is active:

- Add LocationGuard.SessionActive() method
- Check !guard.SessionActive() in GetIntoLoc before using sharky location
- Update tests to verify fallback when inactive and direct read when active

Align execution order in ReplaceLoc with Delete to enforce the principle
of marking resources in LocationGuard prior to releasing them in Sharky:

- Move guard.MarkFreed before sh.Release in ReplaceLoc
- Add test coverage verifying that old locations trigger safe indexStore
  fallback after ReplaceLoc

…oLoc address locking

…ore the field

The location hint added to ChunkBinItem only lands on entries that are
written after the upgrade. Everything already in the reserve stays in the
106-byte legacy layout, Unmarshal leaves its Location zero, and GetIntoLoc
falls back to the retrieval-index lookup for every one of those chunks. The
dual-format Unmarshal makes the old records readable; nothing populates
them, so on an upgraded node the hint is inert until the reserve turns over
on its own, which takes days or never.

Measured on a 2.41M chunk testnet reserve: GetIntoLoc fell back on 100% of
chunks and leveldbstore.Get stayed at 2731 B/chunk, unchanged from the
branch without the hint.

Add migration step 8, which walks the bin index, resolves each address in
the retrieval index once, and writes the location back.

Entries are flushed in windows rather than collected up front: the reserve
holds millions of items and ReserveRepairer's collect-everything approach
would cost hundreds of megabytes while the node is still starting. Since
Location lives in the value and the key stays (Bin, BinID), rewriting does
not disturb the iteration it runs inside. Each window is spread over
NumCPU workers because the per-entry retrieval-index lookup is a random
read and would otherwise leave the disk idle.

The step is idempotent. Entries that already carry a location are skipped,
and entries whose chunk is gone from the chunkstore keep a zero location so
the sampler keeps falling back for them.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_018s5GBDGHX6Nop4RxHhsCr2