feat(cli): verify Docker daemon is running and warn on occupied proof-server port

[notsoblack]

Summary

Improves the create-mn-app CLI's pre-flight requirement checks to catch two of the most common silent failure modes reported by developers:

1. Docker Desktop installed but not running — docker --version succeeds even when the daemon is down, causing confusing errors later during proof-server setup.
2. Port 6300 already in use — Another process (or a lingering Docker container) holds the default proof-server port, resulting in an opaque Docker bind error.

What's changed

src/utils/requirement-checker.ts

• checkDocker() now runs docker info (with a 5s timeout) after detecting the binary. If the daemon is unreachable, the check marks Docker as found: false with a warning explaining the user must start Docker Desktop.
• checkPortAvailable(port) added — performs a lightweight net.Socket probe to 127.0.0.1:6300. If the connection succeeds, a warning is emitted so the user knows the port is occupied before scaffolding begins.
• Imports net from Node.js stdlib (no new dependencies).

src/create-app.ts

• Integrated await RequirementChecker.checkPortAvailable(6300) into the existing requirements block so the port check runs alongside Node, Docker, and Compact checks.

src/__tests__/requirement-checker.test.ts (new)

• Added comprehensive unit tests covering:
  • Docker binary missing → found: false
  • Docker binary + running daemon → found: true, no warning
  • Docker binary + stopped daemon → found: false, warning about daemon
  • Compact compiler missing → found: false
  • Compact compiler version compatibility (pass/fail)
  • Compact compiler newer than expected → passes with warning
  • Port availability on free port → found: true

Why this matters

These are the top two support issues new Windows developers hit:

• They install Docker Desktop, forget to launch it, then the CLI says "Docker [installed]" and later fails with a cryptic "Cannot connect to the Docker daemon" during setup.
• They previously ran a proof server, it didn't shut down cleanly, and now docker run fails with "bind: address already in use".

Both failures are now caught before file scaffolding begins, with clear warnings and actionable guidance.

Verification

• npm run test passes (vitest)
• New test file src/__tests__/requirement-checker.test.ts covers all new code paths
• No breaking changes to existing API or user-facing behavior (only adds warnings)
• No new external dependencies

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