fix(theme): drift-guard test, interactive stylebox states, ThemeKeys font consts, richer showcase (whole-branch review)

- build_game_theme.gd: extract build_theme() as a static func returning the
  fully-built Theme; _init() is now a thin save+quit wrapper. Enables a test
  to build a fresh Theme in memory without running the SceneTree main loop.
- test_theme_resource.gd: add test_committed_tres_matches_builder() — builds
  a fresh Theme via the preloaded builder and compares bg_color against the
  committed game_theme.tres for PrimaryCTA/normal, ParchmentCard/panel, and
  DarkPanel/panel. Catches "palette/builder changed, .tres not regenerated."
- build_game_theme.gd: give Tab, TabActive, and Chip explicit hover/pressed/
  focus styleboxes (focus is StyleBoxEmpty to suppress the default ring) so
  they no longer fall back to Godot's default gray button on interaction.
  Add a disabled stylebox + font_disabled_color to PrimaryCTA for the
  unaffordable-CTA state the 2a shell will need.
- theme_keys.gd: add HEADING/ACCENT/MONO font-role consts (kept out of ALL,
  which is stylebox-variation-only); build_game_theme.gd and
  theme_showcase.gd now reference them instead of raw StringName literals.
- theme_showcase.gd: add an italic DM-voice RichTextLabel (EB Garamond
  italic face, spec §6) and two semantic-coloured chips (BLOOD for
  aggressive actions, GOLD for pay/faith) alongside the existing neutral
  chip, demonstrating the script-applied colouring from spec §5.3.
- dark_bay.gdshader: comment noting the stripe uses FRAGCOORD (screen-space)
  while the vignette uses UV (node-local) — intended for full-bleed use.
- surfaces/*.gd: ## comments documenting the accepted deviation from spec
  §5.4 (uniforms set at runtime from Palette rather than baked into the
  .tscn) — no behavior change.
- Regenerated game_theme.tres via build_game_theme.gd.

Full suite: 125/125 (was 124, +1 drift-guard test).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01GFDm1ku9WDUacK2S78m2be

client/assets/theme/shaders/dark_bay.gdshader

@@ -4,6 +4,11 @@ uniform vec4 stripe_b : source_color;
 uniform float band_px = 16.0;    // 16px bands per README
 uniform float vignette = 0.55;   // edge darkening strength
 void fragment() {
+	// Coordinate spaces differ on purpose: the stripe uses FRAGCOORD (screen-space,
+	// not node-local), while the vignette below uses UV (node-local, 0..1 across
+	// this node's rect). An offset DarkBay instance therefore keeps its stripes
+	// anchored to the screen rather than restarting at the node's own origin —
+	// intended for full-bleed use.
 	// 135deg diagonal stripe: sum of pixel coords, banded.
 	vec2 px = FRAGCOORD.xy;
 	float diag = px.x + px.y;