Document macOS support and harden npm pack checks

[devinoldenburg]

Summary

• Add docs/supported-platforms.md describing the macOS 27.0+ requirement and the fail-fast error text.
• Point the README Requirements section at that doc.
• Declare "os": ["darwin"] in package.json.
• Add npm run check:pack (wired into prepack) so a missing files entry cannot ship a broken tarball.

Depends on #5 for the runtime gate itself.

Test plan

• npm run check:pack
• npm pack --dry-run includes src/macos.js and docs
• CI green after Enforce macOS 27+ before launching fm benchmarks #5 lands (or rebase onto main)

[devinoldenburg]

Self-review:

• Documents macOS 27.0+ requirement and the fail-fast error text.
• Clarifies that doctor still runs on older hosts for diagnostics.
• "os": ["darwin"] marks the package macOS-only on npm.
• check:pack walks runtime imports and verifies the packed tarball (uses --ignore-scripts to avoid prepack recursion).
• Rebased onto main after Enforce macOS 27+ before launching fm benchmarks #5.

Waiting on CI, then merging.