siso: Introduce 'siso fs gc' subcommand for target build label cleanup

Adds the `siso fs gc` CLI subcommand to enable mathematical build label
garbage collection against the HashFS `.siso_fs_state` ledger.

This tool allows developers and continuous integration workers to sweep
orphaned or evicted build target artifacts from disk while preserving
shared libraries and header caches.

Key capabilities:
- Supports positional regex target eviction (`siso fs gc "target-.*"`)
- Supports LRU build cache bounding (`siso fs gc --retain_last_x=3`)
- Protects shared files using bitwise intersection (`(fileMask & keepMask) == 0`)
- Includes `--dry_run` flag for non-destructive simulation and reporting
- Exports structured metrics to `siso_metrics.json` for telemetry pipelines

Also introduces an E2E integration test suite under
`subcmd/ninja/e2etests/fs_gc_test.go` and wires `--build_label`
into `setupBuild` in `utils_test.go`.

This is the 4th CL for go/targeted-build-cleaning.

BUG=446692498
TAG=agy
Change-Id: I096d9ed17b0163142212f96b7d9d40f497abe4ad
Reviewed-on: https://chromium-review.googlesource.com/c/build/+/8183971
Reviewed-by: Yuke Liao <liaoyuke@google.com>
Commit-Queue: Saatvik Sharma <sharmasaatvik@google.com>