Add Duskfen: Level 5 with rising-tide mechanic and dusk theme

[cmalpass]

What

A fifth level, Duskfen: The Drowned Vale — the first level whose environment actively plays against you: a rising tide that climbs 68px over ~136s while the run is live.

• Mechanic — the waterline rises during play (and through knockback/death, so the clock keeps pressure), freezes on victory. Submersion costs a heart and splashes Rex back up; a one-shot "The tide is rising!" ping with a low-whoosh SFX. Water renders in screen space with a gradient + animated sine surface.
• Design — 7600px across 5 sections: dry intro, beetle bog with a lava pit, canopy with spring + spitter + pteros, flooded lowland with a crumble bridge and plate/door gate, and the drowned vale with a continuous elevated "dry hop" route up to the nest rock. At max tide, only ground at or above y≈446 stays dry — the route is tight but fair.
• Dusk theme — twilight palette (deep purple → warm orange horizon), purple moon, stars, ember-dimmed volcano, soft purple clouds, firefly weather (16 blinking glows), and a slow minor-key 76bpm track.
• Victory flourish — "The fen falls silent. The nest is home." + tide swell + drifting particles.
• Lore — 3 new field notes (the waterline / fireflies with a job / the silent fen).

Validation (local gate, per project standard)

• npm run typecheck ✓
• npm test — 264/264 (8 new in tests/dusk.test.ts: registry, level structure, tide advance/cap/freeze, damage+bounce+one-shot ping, reset, dusk plumbing; legacy level-count assertions updated)
• npm run build — 155.56 kB / 47.72 kB gzip single file
• npm run lint ✓
• Headless Playwright E2E — 7/7 (menu → DUSKFEN card → start with tide → waterY decreases → teleport-to-nest victory flourish → tide frozen → zero page errors), screenshots visually verified
• Daily generator untouched: it uses an explicit 3-theme list and never emits tide, so seeded dailies are unaffected