M2.5: ShaderMaterial + texture resolution + fog; multi-skeleton verified

- m2_material: Metin2-style ShaderMaterial. blend_mix shader (opaque /
  alpha-blend / alpha-test-cutout via `mode` uniform) + blend_add shader
  (additive glow). modulate(tex, COORD) * COLOR, diffuse_lambert.
- Metin2Model: material_mode prop ("metin2" ShaderMaterial | "standard");
  _resolve_texture does filename-convention + case-insensitive dir scan of
  *.dds (face/hair keyword slots, stem match, fallback); set_surface_texture
  override; guess_blend heuristic (hair->cutout, cape->alpha, effect->add).
  Texture cache. Fixed opaque path to NOT alpha-discard (was hiding bodies).
- harness: AABB-based camera framing, engine depth fog (MTGODOT_FOG=1),
  MTGODOT_ANIM_T fixed-pose capture (needs explicit anim reload first),
  MTGODOT_AUTOSHOT batch screenshot.

Verified: warrior 5/5 surfaces textured under the shader (t=9 dance pose,
lit, fogged). assassin 17/17 textured via dir scan. shaman_lord (gr2 v7)
renders. Screenshots test/golden/m25-*.png, m2c-t*.png.

Gap (M2.5 gate not closed): libgr2 POD API exposes no material/texture-name/
blend data, so multi-slot -> texture/blend mapping is heuristic only. Needs
a libgr2 material API (gr2_material.cpp walking the type tree) — same gap as
xrender-poc M1 T5. Documented in docs/GODOT-POC-PLAN.md §M2.5 T2.5.6.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01WaHYEY9rwLWt21PULiYjeJ

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