feat(api): map exits CRUD with blocked-tile and topology checks (#10)

[Aduersarius]

Closes #10.

Summary

• Admin API to list (outbound + inbound), create/update, and delete map exits persisted in mapas_source/mapa_<n>/specials.json.
• Strict validation: destination map must exist; destination tile must be in-grid and not blocked (reads terrain.json palette/rows).
• Atomic bidirectional pair creation (validates both landings before writing).
• Topology reports: orphan exits (dest map missing), maps with no inbound links, and BFS-unreachable maps from entry map 1.
• Fixture-based unit tests covering blocked-tile rejection, CRUD/persist, bidirectional pairs, and reachability.

Differentiation vs open PRs

• vs feat(api): map exits management, blocked tile guard and topology audit (#10) #63: in-memory pure functions + kitchen-sink FE/protocol/converter/unrelated services. This PR is scoped to api/, actually reads/writes specials.json / terrain.json, exposes HTTP admin routes, and tests against temp fixtures on disk.
• vs feat(core): implement core game data sync, protocol package, CI matrix, and map toolchain (#28, #11, #27, #23, #10, #20) #74: multi-issue bounty batch. This PR is single-issue and reviewable on its own.

Verification

• Tests: api/src/tests/map-exits.unit.test.ts (temp dir fixtures; no DB required).
• Local install/tsc blocked in agent session; CI should cover compile/test.
• No txs/keys/funds involved.

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