Extend track schema with four directed circuit gates #107

Parent: #105

Objective

Introduce a versioned track definition that stores directed start, finish, pit-entry, and pit-exit lines for every circuit layout.

Scope

• Define one reusable bounded directed-gate structure with left/right GPS endpoints, expected crossing heading/tolerance, minimum crossing speed, and rearm-corridor distance.
• Store all four required gates plus a track-level minimum plausible lap time.
• Permit start and finish to use identical geometry.
• Project every endpoint into the existing local metre coordinate system.
• Reject missing, degenerate, out-of-range, oversized, or unknown geometry fields.
• Update JSON Schema, checked-in fixtures, parser diagnostics, hashing, documentation, and host validators.
• Retain one JSON file per circuit layout and the 4096-byte bounded parser contract where feasible.

Acceptance criteria

• A valid four-gate file loads allocation-free and exposes geographic and local endpoints for every gate.
• Any missing or invalid gate fails without modifying the active track.
• Start/finish identity is accepted; degenerate individual lines are rejected.
• Existing schema behavior has an explicit migration or unsupported-version policy.
• Targeted parser/validator tests and full build gates pass.

Related: #34.