Phase 6 done: see-through cores, faint on sacrifice and rebirth; Phase 7 design

Adrian (2026-11-02): the cores should be transparent so the Pokémon
shows; the Pokémon's death sound when a sacrifice is made or a rebirth
ends; then start the next phase (Phase 6 approved).

- Cores drawn as a solid frame around tinted glass, rendered
  translucent, no occlusion (tools/machine_assets.py).
- The shown Pokémon plays Cobblemon's faint animation and its cry
  pitched down (Cobblemon has no death sounds), vanishes after 2 s; a
  reborn Pokémon then shows in its new form. Tests check the new form.
- Phase 7 decisions (every guardian gets area attacks, Groudon for the
  Basalt Furnace, Giratina replaces the Ender Dragon, dying or leaving
  the arena loses) and design in docs/design.md; README status.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

README.md

@@ -2,7 +2,7 @@
 
 A free NeoForge 1.21.1 modpack on Cobblemon 1.8: one large island with soft-gated, Palworld/ARK-style progression, plus a companion mod for the mechanics no addon covers. The plan is in [docs/roadmap.md](docs/roadmap.md); decisions taken so far are in [docs/design.md](docs/design.md).
 
-**Status:** Phases 0–5 approved (2026-10-31); Phase 6 (Pokémon machines) in progress. Checks per phase: `docs/checklists/`.
+**Status:** Phases 0–6 approved (2026-11-02); Phase 7 (boss battles with area attacks) in progress. Checks per phase: `docs/checklists/`.
 
 ## What's in the pack