soldr#1006 Lane 3. Closes the easiest remaining gap from the meta:
\`x86_64-apple-darwin\` is now built from \`ubuntu-latest\` via the same
\`mac_cross_linux: true\` template branch the arm64 lane already uses
(landed in #771). Soldr's apple_sdk fetcher auto-picks the
\`darwin-x86_64\` thin shape when the target triple matches, and
\`cargo zigbuild --target x86_64-apple-darwin\` resolves SDKROOT +
frameworks identically.

Net effect:
- fbuild's release matrix gains a 5th target (mac Intel) for the same
  per-release runner cost (just adds another ubuntu-latest job).
- Zero \`macos-latest\` runners left in the matrix — both Apple
  targets cross-compile from Linux.

## Docker harness changes

* Renamed \`ci/docker-mac-arm64-cross/\` → \`ci/docker-mac-cross/\` since
  the same image now validates both Apple targets.
* \`build.sh\` takes a target as positional arg or \`TARGET=\` env var
  (default: aarch64-apple-darwin); per-target arch verification —
  \`arm64|aarch64\` for arm64 lane, \`x86_64\` for Intel lane.
* Image tag bumped from \`fbuild-mac-arm64-cross\` →
  \`fbuild-mac-cross\` to match.

## Test plan
- [x] Build docker image
- [ ] Run \`bash ci/docker-mac-cross/build.sh x86_64-apple-darwin\`,
      verify 3 Mach-O x86_64 artifacts under staging/
- [ ] Trigger a release via version bump; verify new matrix entry
      ships \`fbuild-v<ver>-x86_64-apple-darwin.tar.gz\`

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>