feat: bigger, layered, animated pets with state-driven auras

[dropdevrahul]

Summary

A real visual upgrade for all four pets. Before/after comparison below.

• Bigger canvas: HL 8 → 12, standard width SW = 22. New padAt(rows, topRow) helper lets each layer paint into a specific row band so the sprite composes cleanly: aura at the top, eyes/body in the middle, ground shadow at the bottom.
• Per-state auras: a new core/pets/effects.ts provides auraFor(state) — sparkles for happy, drifting Zzz for sleeping, hearts for excited, droplets for sad, crumbs for eating, drift specks for playing, a rare twinkle for idle. Every pet × every state gets the matching aura layered above the body.
• Ground shadow under every pet via a shared groundShadow() layer — small thing, makes the pet feel anchored.
• Layered idle for every pet (cat already had it, the others didn't): body / eyes / accent on independent timers, so motion is organic instead of all ticking together.
  • Cat: ear-twitch + flicking tail accent peeking from the right.
  • Hamster: cheek pouches puff in and out (° → ◦).
  • Ghost: wavy ectoplasm tail shifts left/right.
  • Robot: antenna LED pulses + chest module heartbeat.
• Richer silhouettes using box-drawing chars (╱ ╲ ┌ ┐ └ ┘ │ ─ ●) instead of bare ASCII.
• Snapshot tests lock in canvas geometry: every layer frame is exactly HL × ≥SW, every step has a duration, the frame-0 composite is HL rows.

Before / after (cat happy)

before                              after
  /\_____/\                            ✦           ✧
 /  ^   ^  \                                ·         ·
(  == ω ==  )                           /\_____/\
 \  '-'  /                             (  ^   ^  )
 (__)  (__)                            |  ●   ●  |
                                       |    ω    |
                                       |  \___/  |
                                        \       /
                                         |_____|
                                        (__) (__)

                                       ▁▁▁▁▁▁▁▁▁▁▁▁

Run bun scripts/preview.ts to see all 28 (pet × state) combinations.

Test plan

• bun test → 123 pass (38 existing + 85 new sprite snapshot tests, 0 fail)
• bun run typecheck → 0 errors
• bun scripts/preview.ts — every pet × every state renders at the new geometry
• staticSprite() via core/render.ts produces correctly-shaped frames for the statusline / inline ASCII card
• Reviewer: run campy watch and switch through each pet to eyeball the layered motion

Out of scope (good follow-ups)

• Per-pet multi-color rendering (touches core/render.ts color path)
• Rounded Unicode speech bubbles (touches core/render.ts bubble path)
• New pet types