Add the entities.png manifest, paste-in template and sprite loader

The sheet becomes a fixed grid (one row per entity, one column slot per
animation state) described by a sidecar entities.toml that only maps
names to rows; the loader reads present states and frame counts off the
pixels, so assembling the sheet by hand needs no bookkeeping.
tools/mktemplate.py renders the labelled template with low-alpha slot
guides. The CGA atlas is kept for larger UI text.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

game/Cargo.toml

@@ -7,3 +7,5 @@ edition = "2024"
 sim = { path = "../sim" }
 pbio = { git = "https://git.bnd.wtf/irrlicht/pbio.git", branch = "truecolor16" }
 png = "0.18.1"
+toml = "0.9"
+serde = { version = "1", features = ["derive"] }