Unify the Godot 4.7.2 CI environment and cache it

Update project features and CI installs/templates to Godot 4.7.2, remove the
old version matrix, and cache the Godot binary and export templates with Gitea
Actions cache. Update release/docs/version references accordingly. Project
import and headless tests pass locally under 4.7.2.

PLAN.md

@@ -159,7 +159,7 @@ Primary decisions: [ADR-0007](docs/adr/0007-locomotion-and-input-layers.md), [AD
 
 ## Tech Stack
 
-- **Engine:** Godot 4.3 baseline (local dev verified with 4.7.2), GL Compatibility renderer (Web parity); optional Forward+ Desktop HQ build.
+- **Engine:** Godot 4.7.2, GL Compatibility renderer (Web parity); optional Forward+ Desktop HQ build.
 - **Language:** GDScript.
 - **Architecture:** venue-driven simulation; shared systems plus `Resource`-based scenario data.
 - **Navigation:** `NavigationRegion3D` / `NavigationAgent3D`.