fix(theme-guard): close the drift guard's third miss — the meta-guard's own continue whitelisted the gap

test_committed_tres_matches_builder has now been "fixed" three times (traps.md #12).
v1 hardcoded 3 checks; v2 iterated ThemeKeys.ALL but missed the six font roles; v3
added a meta-guard specifically to catch "the builder styles a thing guarded by
nothing" — but that meta-guard contained `if fresh.get_type_variation_base(variation)
== &"": continue`, which whitelisted exactly the case it was built to catch. The
builder styles RichTextLabel directly (build_game_theme.gd's _rich_text()), with no
set_type_variation, so it reports base "" and was skipped unconditionally — and
RichTextLabel is what renders every line of DM prose the creation screen shows
(_origin_text, _detail_blurb). Nothing compared its fonts, font size, or colour, nor
the theme-level default_font/default_font_size, against the committed .tres.

Adds ThemeKeys.BASE_TYPES for base Control types the builder styles directly, folds
it into the drift guard's coverage, replaces the meta-guard's continue with an
assertion that names the offending type, and adds the missing theme-level default
font/size comparison. Proved with three reverted breaks: a RichTextLabel font-size
edit, a theme-level default_font_size edit, and an unregistered "GhostRole" variation
— all three now go red and name the drifted property.

Also: strengthens test_creation_copy's error-copy sweep to assert a mapped error
actually reaches its authored line rather than silently falling through to the
generic UNSPOKEN fallback (CreationCopy.UNSPOKEN satisfied all four prior checks,
so a renamed validator string could regress silently); and removes two assertions
that were true by construction / already true before their test's action ran.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

client/scripts/theme/theme_keys.gd

@@ -47,6 +47,19 @@ const FONT_ROLES := {
 	SECTION_LABEL: "Label",
 }
 
+## Base Control types the builder styles DIRECTLY — no set_type_variation is
+## ever called for these, so a fresh Theme reports get_type_variation_base("")
+## for them. Kept as its own set (not folded into ALL or FONT_ROLES) because a
+## base type is not a variation: nothing sets `theme_type_variation` to
+## "RichTextLabel" anywhere, the node's own class name is the lookup key.
+## Today: RichTextLabel, the builder's prose voice (mock README Typography:
+## serif body/italic emphasis) — the creation screen's origin panel and
+## calling detail panel are RichTextLabels rendering this styling directly,
+## i.e. every line of DM prose the screen shows.
+const BASE_TYPES := {
+	&"RichTextLabel": true,
+}
+
 ## Every variation name + the base Control type it decorates. The builder and the
 ## test both iterate this so they can never drift apart.
 const ALL := {