[M3-S2] Annotate the Hidden Garden collision #85

Problem

The prologue walk needs ground, a log to jump, a slope to climb, the bench, Dexter's stops and the pond trigger.

Acceptance criteria

• .collision.json per Hidden Garden chunk
• Dexter's waypoints and the pond trigger as purple interactables
• Gentle verticality only: hills, the log, the deck

Notes and dependencies

Colours: 04. Suggested owner: Lex (level design, collision, encounters).

Verification

Overlay screenshots in the PR; walkable on the preview.