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.

assets/sprites_ph/slug_ph.png.import

@@ -0,0 +1,40 @@
+[remap]
+
+importer="texture"
+type="CompressedTexture2D"
+uid="uid://bnbsde6j7gwsi"
+path="res://.godot/imported/slug_ph.png-c68013faba14e2a7a9a40558a57d8215.ctex"
+metadata={
+"vram_texture": false
+}
+
+[deps]
+
+source_file="res://assets/sprites_ph/slug_ph.png"
+dest_files=["res://.godot/imported/slug_ph.png-c68013faba14e2a7a9a40558a57d8215.ctex"]
+
+[params]
+
+compress/mode=0
+compress/high_quality=false
+compress/lossy_quality=0.7
+compress/uastc_level=0
+compress/rdo_quality_loss=0.0
+compress/hdr_compression=1
+compress/normal_map=0
+compress/channel_pack=0
+mipmaps/generate=false
+mipmaps/limit=-1
+roughness/mode=0
+roughness/src_normal=""
+process/channel_remap/red=0
+process/channel_remap/green=1
+process/channel_remap/blue=2
+process/channel_remap/alpha=3
+process/fix_alpha_border=true
+process/premult_alpha=false
+process/normal_map_invert_y=false
+process/hdr_as_srgb=false
+process/hdr_clamp_exposure=false
+process/size_limit=0
+detect_3d/compress_to=1