macOS (Apple Silicon) support #51

First feature after v0.1.0; becomes 0.2.0. Apple Silicon (arm64) only: x86_64
macOS is not claimed, and the reactor, fuzzing and TSan stay Linux-only.
Depends on the assembly macro issue (#) so the .S files already carry the
Mach-O branch.

Already in place: arch normalisation (arm64 -> aarch64, freestanding only on
arm), make.sh accepts Darwin and detects a native build, the ASan gate is
the CFIBER_ASAN_ENABLED build define with a __has_feature mismatch check,
cfiber_init zeroes the context, and the growable-stack test already catches
SIGBUS (what macOS raises on the guard page). No local macOS machine: parts
below are proven by the GitHub job, expect a few CI round trips.

A. Growable stacks on Darwin

• MAP_STACK is undefined on macOS: define it to 0 when missing. MAP_NORESERVE
  exists and is ignored.
• MADV_DONTNEED does not drop resident pages on Darwin; use MADV_FREE_REUSABLE
  (or MADV_FREE) under APPLE in cfiber_growable_stack_recycle.
• 16 KiB pages: guard page and growth granularity are four times larger. Code
  and tests already use sysconf(_SC_PAGESIZE); document, do not special-case.
  One Darwin sentence in the overcommit paragraph of docs/memory.md.

B. Build and script

• C23 constexpr needs Apple Clang 17 (Xcode 16.3+) or Homebrew LLVM. If the
  runner's Xcode is too old: brew install llvm, CC pointed at it, and a line
  in docs/building.md.
• CFIBER_BUILD_SHARED yields libcfiber.dylib with SOVERSION; confirm install +
  find_package consumer once (DYLD_LIBRARY_PATH).
• make.sh: drop "(macOS is unverified)" from the host check once CI runs.
  timeout is only needed for QEMU, so native runs need no coreutils.
• Apple arm64 ABI: x18 reserved and never touched; frame pointers mandatory,
  a fresh fiber starts with x29 = 0; 16-byte alignment already holds. Listed
  so the port does not re-check them.

C. CI and docs

• .github/workflows/macos.yml on macos-15 (arm64), no container: make.sh -t -e and -t -e -d, then --asan and --shared once green. Same paths:
  filter and timeout-minutes as the other workflows; README badge.
• README platform table row: macOS arm64 | AAPCS64 (Apple) | Native (GitHub macos-15 runner); adjust "No other host is supported".
• docs/building.md requirements; docs/memory.md Darwin notes.
• CHANGELOG Unreleased -> 0.2.0 and project(cfiber VERSION 0.2.0) in the tag
  preparation PR.

Check

• Linux matrix unchanged.
• macOS job green: register set and FP control, return hook, allocators,
  scheduler, fixed and growable stacks including the guard-page fault, death
  tests, scheduler example.