Optimize native exact-tree materialization (replacement)

[kvkenyon]

Successor to the preserved predecessor #28. This replacement branch starts at current origin/main (f29da812) and imports only its RTCGit performance, timeout, fixture, documentation, and focused-test changes; the predecessor branch and PR remain untouched.

The change batches committed blob metadata/content, reads numstats once, and bounds patch concurrency at four while retaining exact revision identity, shell-free /usr/bin/git, option termination, hostile filename bytes, cancellation checks, error mapping, and existing file/patch/total/time caps. It also uses pinned-Git-compatible -z request framing and newline response parsing.

The first two unsigned-validation attempts reached the workflow's 15-minute timeout while RTCIngestTests was running. Four concurrent patches had placed blocking pipe reads and waitpid calls on Swift's cooperative executor, which can starve on lower-core CI hosts. The runner now performs those bounded blocking calls on an overcommitting dispatch queue; there was no workflow concurrency rule or superseding cancellation.

Validation:

• Focused CLT smoke: batch framing/EOF, timeout mapping, hostile filename byte fidelity, committed-tree reads, dirty-tree exclusion, and exact SHAs passed.
• Single-lane cooperative-pool stress: three measured 101-file materializations completed in 4.4 seconds total with p95 956.72 ms.
• 101-file/5,000-line release benchmark on Apple M5, 24 GiB, macOS 26.1, Swift 6.2.1, Apple Git 2.50.1: p50 1,067.98 ms, p95 1,207.36 ms, 109 Git processes/iteration (20 iterations, two warmups).
• npm run check passed.
• scripts/native-check.sh built the native graph and passed checks through the known unrelated RTCIngestTests abort: RTCIPC.IPCTransportError.socketPathTooLong from this long isolated worktree. Remaining native executables and Xcode project generation passed when run directly; the branch does not broaden into RTCIngest/RTCIPC ownership.

XCTest targets, the full-Xcode build/UI tests, signing, notarization, clean-install, and release evidence remain unclaimed on this Command Line Tools host.