docs(agents): macos min-version — final state (shipped & verified)

[Sunrisepeak]

Doc-only. Brings the macOS min-version design doc to its final state:

• Status header → SHIPPED & VERIFIED (mcpp v0.0.50 + xlings v0.4.50, floor 14.0).
• §4 + landing lessons from PR feat(0.4.50): macOS releases run on 14.0+ (static libc++ via mcpp 0.0.50) #319's four CI rounds: three-layer version pins (incl. the hidden .xlings.json workspace pin), host-tool runtime-lib-path poisoning in test fixtures, newest-binary selection for release assertions.
• §5 resolver TODO status → open as mcpp PR core/xim: add types(template/script) features for xpkg and optimize p… #119; fixed a duplicated table header.
• New §6: final verification matrix results (macos-14/15 against real release artifacts) + distribution sync record.