Mythic Masters: per-field looks for the big moves, a field-coloured guard, and the particle cap on a phone #2820

Part of #2802 (the effects front, round 2), after PR #2810. One worker, from main. Owns apps/wildclash/src/fx.ts, src/model.ts, src/main.ts and test/fx.test.ts. The animation-side round 2 is #2809, in other files.

• Per-field looks for the big moves. A field's grain (dot, shard, chunk, leaf, streak) and colour do all the work now. Give each field's nova, wave, rain and rampage something of its own: a frost nova leaves a rime ring on the ground that lasts a few seconds, a grove wave sprouts leaves where it passes, a storm rain forks (streaks that split), an iron rampage throws sparks off the ground, an ember breath scorches a fading mark, a tide wave leaves a wet sheen, a shadow nova dims the aura of the one it hits.
• The guard is still the capsule model's white ring (creatureModel in src/model.ts). Draw a field-coloured disc or shell on the body in fx.ts while state === "guard", with a flash when a hit is blocked, and delete creatureModel if nothing else needs it (the world view may; check src/world/view.ts).
• Phone check. MAX_LIVE is 700. Run a busy fight (two bruisers, ?watch=1) at a phone size and on a throttled CPU; if it stutters, lower the cap or merge the motes into one InstancedMesh per shape.

Judge with a fight per archetype side-on (window.wildclash.camera("side")). Before and after screenshots, scored out of 10, in the PR. No sheet, stat or move-data changes.