Persist actionable raw helper crash diagnostics

[lynnswap]

Purpose

Preserve bounded diagnostics that explain raw-helper crashes and provide enough verified process identity to correlate Simulator failures with Crash Reporter.

Closes #81.

Changes

• replace the last-eight-lines tail with terminal-safe, byte-bounded combined output that retains both the diagnostic prefix and termination tail
• add an invocation-bound startup handshake with the actual helper PID, executable name, LC_UUID, and start timestamp before target loading begins
• build one canonical failure capsule with typed wrapper or child termination, bounded diagnostics, helper identity, and termination-observation time
• recognize Simulator child signals only from the exact final simctl line corroborated by the 128 + signal wrapper exit status
• persist the capsule unchanged through the existing runTargets.failureSummary path and keep successful typed diagnostics separate
• read handshake and diagnostics reports through one descriptor-bound, no-follow, regular-file-checked, maximum + 1 bounded reader
• share current-process Mach-O UUID resolution between Tooling and raw helpers

Testing

• DEVELOPER_DIR=/Applications/Xcode_27.app/Contents/Developer swift test --force-resolved-versions
• PrivateHeaderKitCLI suite: 90 tests passed with Xcode 27 / Swift 6.4 and Xcode 26.5 / Swift 6.3.2
• iOS 27 and watchOS 27 Simulator helper release builds passed
• bash Scripts/test-release-scripts.sh
• exact iOS 27.0 beta 24A5390f runtime reproduction for AXSpringBoardServerInstance
  • retained exception name/reason, first frames, omission marker, and terminal frames in 17 lines / 1,725 bytes
  • recorded child_signal(6), wrapper status 134, actual PID, LC_UUID, helper start, and observed termination
  • matched Crash Reporter PID, LC_UUID, helper name, and SIGABRT/code 6
  • restored the runtime override and deleted the run-owned Simulator
• codex-review against main: no findings (job C82A0657-5269-41C7-8546-A66C0BBD0440)

Screenshots are not applicable for this CLI-only change.