initial: world-overlay v0.1.0 — transient world-space UI primitives

Generic world-space transient UI lib. Spawns text, icon, rect, rect_lines,
and composite overlays anchored to entities or fixed world positions.
TTL-based expiry with fade-in/hold/fade-out alpha envelope and optional
upward rise. Camera-aware world-to-screen transform. Entity-destroyed
fallback keeps last_pos snapshot. Five test backdoors for headless testing.

manifest.lib

@@ -0,0 +1 @@
+{"id":"lib-core.world-overlay","version":"0.1.0","api_min":"0.1","deps":[]}