docs: reconcile README/ROADMAP/PLAN state and settle slotmap vs String IDs

- README stays the source of truth for current state; drive/item-5 now
  reflects the settled decision: String IDs for the MVP, slotmap deferred.
- ROADMAP gains a 'point de depart' (present-state) block, marks glam done,
  removes the slotmap Phase-1 mandates, and documents the String-ID decision
  plus the deferred 'typed handles' step.
- PLAN corrects stale [X] checkmarks (App render() cannot draw, Scene
  rendering not automated, simple example does not compile) and notes its
  verification checklist against current reality.
- Remove the unused slotmap direct dependency from wsg-lib (package remains
  in Cargo.lock only as a transitive dep of glow).

lib/Cargo.toml

@@ -12,7 +12,6 @@ winit = "0.29"  # For window management — pinned to match examples
 thiserror = "2"
 bytemuck = { version = "1.25.0", features = ["derive"] }
 glam = "0.33"
-slotmap = "1.0"
 
 [dev-dependencies]
 pollster = { version="0.4.0", features = ["macro"] }