feat(ui): add YAML-based UI string loading system (#409)

Adds UIStrings autoload that loads display text from a YAML file,
replacing hardcoded strings in HUD and interaction prompt. Copy team
can now author UI microcopy in client/data/ui-strings.yaml without
touching GDScript. Includes 38 strings across 5 categories and 16
gdUnit4 tests for the parser and lookup API.

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

client/data/ui-strings.yaml

@@ -0,0 +1,51 @@
+# UI strings for The Settled Reach client
+# Authored by copy team. Loaded by UIStrings autoload.
+# Format: one-level-nested key-value pairs (section.key -> value)
+
+interaction:
+  prompt_prefix: "E"
+  talk: "Talk"
+  examine: "Examine"
+  observe: "Observe"
+  pick_up: "Pick up"
+  use: "Use"
+  inspect: "Inspect"
+  open: "Open"
+  close: "Close"
+  interact: "Interact"
+
+hud:
+  mode_label: "Mode"
+  time_label: "Time"
+  health_label: "Health"
+  pause_indicator: "PAUSED"
+  location_prefix: "Location"
+
+knowledge:
+  header_known_contacts: "Known Contacts"
+  header_persons_of_interest: "Persons of Interest"
+  header_case_notes: "Case Notes"
+  header_evidence: "Evidence"
+  header_known_locations: "Known Locations"
+  header_recent_observations: "Recent Observations"
+  empty_panel: "Nothing recorded yet."
+
+relationship:
+  unknown: "Unknown"
+  acquaintance: "Acquaintance"
+  colleague: "Colleague"
+  trusted_colleague: "Trusted colleague"
+  informant: "Informant"
+  suspect: "Suspect"
+  hostile: "Hostile"
+  friendly: "Friendly"
+  flagged_by_case: "Flagged by case file"
+  person_of_interest: "Person of interest"
+
+tutorial:
+  welcome: "Welcome to Sova Transit District."
+  movement: "Use WASD to move."
+  interact_hint: "Press E near someone to interact."
+  perception_hint: "Press TAB to toggle perception mode."
+  pause_hint: "Press SPACE to pause."
+  look_around: "Look around. Get your bearings."