siso: Introduce Garbage Collection Engine

Adds `hfs.GarbageCollectBuildLabels(ctx, BuildLabelGCOptions)` to the Siso HashFS
backend. This applies mathematical bitwise eviction modeling based on 4
sweep strategies:
- RetainLastX (LRU cache bounding)
- OlderThan (TTL bounding)
- EvictLabels (Explicit target destruction via regex)
- KeepLabels (Explicit target retaining via regex)

It iterates over the internal dictionary ledger mapping and executes a
bitwise sweep against evicted vs kept targets to deterministically
compute cache overlap protection across shared files.

This engine cleanly separates mathematical evaluation from actual OS
execution (`opts.DryRun`), which will allow downstream CLI
layers to retrieve metrics.

This is the 3rd CL for go/targeted-build-cleaning.

Change-Id: Ie323cfaafb18862b79080537669c0d65542dbd2c
BUG=446692498
TAG=agy
CONV=ff3ec10c-40be-426f-b702-052ff5a38e57
Reviewed-on: https://chromium-review.googlesource.com/c/build/+/8141379
Reviewed-by: Yuke Liao <liaoyuke@google.com>
Commit-Queue: Saatvik Sharma <sharmasaatvik@google.com>