feat: wire _ph placeholder sprites into actors (mech, grunt, burster, slug, scrap)

Operator asked for placeholder sprites for playtest feel. Actors now
render the hand-authored pixel maps (tools/aseprite/maps -> assets/
sprites_ph, committed) via code-created Sprite2D — no scene texture
UIDs to hand-author. Dynamic feedback preserved: grunt/burster heat
tint + hit flash drive sprite.modulate, heat-aura ring stays a _draw
overlay; salvage fade drives modulate.a. Arena floor/walls, HUD, and
FX stay primitives. CLAUDE.md greybox-art note updated (was 'no
sprites'); real art (watercolor scenery + pixel sprites) still later.

Smoke gate 3/3 green on nh3-dev headless 4.7.1.

scenes/burster.tscn

@@ -15,6 +15,7 @@ contact_damage = 3.0
 body_radius = 11.0
 base_color = Color(0.6, 0.4, 0.25, 1)
 salvage_value = 3
+sprite_path = "res://assets/sprites_ph/burster_ph.png"
 self_heat_rate = 0.5
 detonation_damage = 2.5
 detonation_mech_damage = 15.0