feat(design-explorer): rulebook binding + prose→claims extraction

Milestone #251 step 2 (#2259). The half of the drift panel that needed to be
testable, which is why it is Python: the frontend has no test runner, so the
fiddly extraction lives server-side and the browser only does set arithmetic
over live token values.

BINDING. A per-user setting `design_rulebook_id` names the rulebook that
describes this install's design system. A setting rather than a column: no
migration, discoverable in the Settings UI (rule #25), and honest about being a
per-install choice rather than a property of the rulebook. No rulebook
designated returns an empty set with rulebook_id: null — the NORMAL case for any
install but the one that set it up (rule #115), which the client renders as an
explanatory empty state rather than an error. The id comes back alongside the
list so "not designated" and "designated but empty" stay distinguishable.

EXTRACTION. No NLP. Rule statements are prose written for humans and should stay
that way, so this takes only what is unambiguous in any prose — the hex colours
and custom-property names a rule mentions. Anything subtler needs a rule author
to opt into a structured form, deliberately left for when someone wants it.

Three things earn their complexity:

- SENTENCE-SCOPED NEGATION. A rule routinely states what the palette requires and
  what it forbids in consecutive sentences ("Parchment #E8E4D8 …, Vellum #C2BFB4
  …. Pure white #FFFFFF is NEVER used."). Detecting negation across the whole
  statement would mark the required colours as forbidden — inverting the finding
  rather than missing it, which is worse. Per sentence, all four come out right.

- HEX NORMALISATION is load-bearing, not tidiness. The rulebook writes #FFFFFF
  and components write #fff; if those don't compare equal the largest drift
  finding in the codebase — 67 hardcoded white text colours (#2275) — reads as
  zero. Alpha forms keep their alpha, since #fff and #ffff are different colours
  and collapsing them would manufacture equality.

- SLASH SHORTHAND. Rulebooks write token families as --fs-radius-sm/md/lg/xl and
  --fs-obsidian/iron/slate/pewter. Both expand under one rule — prefix is
  everything up to and including the LAST hyphen of the first segment — which
  also handles --fs-dur-fast/base/slow. Verified against the real rule text: 18
  tokens from three different shorthand shapes.

how_to_apply is read alongside statement, because rulebooks routinely keep the
statement declarative and put the concrete values in how_to_apply; ignoring it
would miss the checkable half.

Claims dedupe on (kind, value), first source winning, so a colour named by
several rules is one expectation attributed to the rule that introduced it.
Prose with nothing checkable yields nothing — most rules are judgement, not
specification, and a panel that reported unparseable rules as problems would be
unusable.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01UaYUaouG9jjhATyuxCKrQs

src/scribe/app.py

@@ -26,6 +26,7 @@ from scribe.routes.profile import profile_bp
 from scribe.routes.knowledge import knowledge_bp
 from scribe.routes.rulebooks import rulebooks_bp
 from scribe.routes.plugin import plugin_bp
+from scribe.routes.design import design_bp
 from scribe.routes.trash import trash_bp
 from scribe.routes.dashboard import dashboard_bp
 from scribe.routes.systems import systems_bp
@@ -89,6 +90,7 @@ def create_app() -> Quart:
     app.register_blueprint(knowledge_bp)
     app.register_blueprint(rulebooks_bp)
     app.register_blueprint(plugin_bp)
+    app.register_blueprint(design_bp)
     app.register_blueprint(trash_bp)
     app.register_blueprint(dashboard_bp)
     app.register_blueprint(systems_bp)