Add COW storage lifecycle diagnostics

[adamziel]

What it does

Adds forkpress storage compact for macOS APFS sparsebundle-backed COW sites and expands forkpress storage status with branch/storage roots, lock paths, branch counts, and interrupted-operation leftovers.

This is stacked on #14.

Rationale

Sparsebundle-backed sites need a single ForkPress-managed way to stop the site, detach storage, reclaim free image bands, and explain what is still attached or left behind. Direct hdiutil/rm -rf workflows are too easy to get wrong while a server or branch mutation is active.

Implementation

storage compact stops this site's server unless --keep-server is passed, detaches sparsebundle storage, runs hdiutil compact, and leaves storage detached until forkpress serve or forkpress storage mount.

The COW lifecycle now uses:

• operations.lock for branch/storage mutations and request exclusion.
• lifecycle.lock so background server startup and detach/compact cannot race.
• A locked server registry so read/filter/write operations cannot clobber a concurrently registered server.

The COW E2E now checks lifecycle diagnostics and exercises storage compact; macOS CI runs that path with FORKPRESS_FORCE_MACOS_APFS_SPARSEBUNDLE=1.

Testing instructions

FORKPRESS_RUNTIME_BUNDLE=/dev/null cargo test -p forkpress
make test-all
cargo build --release --target x86_64-unknown-linux-musl -p forkpress
tests/test_cow_strategy_e2e.sh target/x86_64-unknown-linux-musl/release/forkpress

[JanJakes]

Reviewed and recorded follow-up notes under the PR #15 section in issue #30: #30 (comment)