feat(client): add interaction prompt system (#405)

Server-driven interaction prompt that displays "E - Talk" when near
an interactable NPC. Decodes v4 nearby_interactions from snapshot,
stores in GameState, renders via InteractionPrompt UI with fade
animation. Extensible interface (get_interaction_target/get_selected_verb)
for future radial verb menu (v0.2).

- Protocol: decode nearby_interactions array with nested VerbOption structs,
  entity relationship/observation fields, tick_rate in GameTime
- GameState: store/clear nearby_interactions per snapshot
- SimBridge: test mode generates v4 format with structured verbs
- InteractionPrompt: PanelContainer with fade in/out, polls GameState
- Tests: 19 new test cases covering protocol, state, sim bridge, UI, encoding
- Fixture assertions updated for v4 protocol version

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

client/tests/test_rendering.gd

@@ -175,7 +175,7 @@ func test_sim_bridge_test_snapshot_has_v2_fields() -> void:
 	SimBridge.reset_test_state()
 	var snap = SimBridge._test_snapshot()
 	assert_that(snap.has("version")).is_true()
-	assert_that(snap.version).is_equal(2)
+	assert_that(snap.version).is_equal(4)
 	assert_that(snap.has("game_time")).is_true()
 	assert_that(snap.has("player_facing")).is_true()
 	assert_that(snap.has("visible_tiles")).is_true()