docs: add Map Builder import-boundary note

[karolswdev]

Goal

Add a short note to docs/player-guide.md explaining the Map Builder boundary.

Pointers

• Product boundary: pm/roadmap/serfbound/phase-42-map-builder/current-phase-status.md.
• Custom-map legal posture: pm/roadmap/serfbound/adoption/asset-and-legal-boundary.md under the custom maps addendum.
• Engine proof: tests/ci/engine-map-editor.test.mjs.

Acceptance

• Explains that the editor is import-gated like playing because it uses the player's decoded runtime art.
• Explains that a custom map can be played locally without becoming an original asset payload.
• Runs npm run test:docs.

No original DOS/Amiga data is needed for this task.