feat(design-systems): import a design system out of a rulebook's prose

Milestone #254 step 3 (#2288). Reuses #251's prose extractor as the reader and
adds the part that makes it an import rather than a list of claims.

**The join is the whole trick.** A rulebook states a design system in two places
and neither half is a token: one rule names the colours ("Obsidian #14171A (page
bg, deepest surface)"), another names the custom properties
(`--fs-obsidian/iron/slate`). The import pairs them on the word — `--fs-obsidian`
ends with `obsidian` — which is the only reason it produces something usable
instead of seventy empty names. The parenthetical becomes the token's purpose,
which is the field a bare hex could never carry.

**Prohibitions arrive as replacements, per the operator's reframe.** Rule 52
declares Parchment and forbids pure white in one breath, so the import emits
"write --fs-parchment instead of #ffffff" — the same fact stated forwards. It
attaches to the FIRST token that rule supplied a value for, not to every token
of that rule, because claiming Vellum is also the replacement for white would be
putting words in the rulebook's mouth.

**A token the rulebook names but states no readable value for is still
proposed, with an empty value.** Radius steps and type sizes are prose ("Small
4px") and nothing here parses them; inventing a parse per shape would be
guessing. The name is real and the value needs a human, so the proposal says
exactly that — and the UI leads with the COUNT of those, because an import that
hid them would look more complete than it is.

Preview is the default on both surfaces and in the UI. An import is a proposal:
rulebooks are written aspirationally and some of what they describe was never
built, so every entry carries the rule id and the sentence it came from and a
reviewer can check the claim rather than trust it.

Existing token names are never overwritten. A value already in the record was
put there deliberately — most likely correcting this importer — so a re-run
fills gaps and lists the rest as skipped, which also makes it safe to repeat.

Colours the rulebook names but never exposes as a custom property produce no
token: it never asked for one, and inventing a name would put something in the
record no rule sanctions.

src/scribe/routes/design_systems.py

@@ -114,6 +114,30 @@ async def resolve_design_system(design_system_id: int):
     })
 
 
+@design_systems_bp.post("/design-systems/<int:design_system_id>/import")
+@login_required
+async def import_design_system(design_system_id: int):
+    """Seed a design system from a rulebook's prose.
+
+    `{"rulebook_id": N}` previews; add `"apply": true` to write. Preview is the
+    default because an import is a PROPOSAL — rulebooks are written
+    aspirationally and some of what they describe was never built.
+
+    Existing token names are never overwritten, so a second run fills gaps and
+    reports the rest rather than undoing corrections.
+    """
+    data = await request.get_json() or {}
+    rulebook_id = data.get("rulebook_id")
+    if not isinstance(rulebook_id, int) or rulebook_id <= 0:
+        return jsonify({"error": "rulebook_id is required"}), 400
+    report = await ds_svc.import_from_rulebook(
+        _uid(), design_system_id, rulebook_id, apply=bool(data.get("apply")),
+    )
+    if report is None:
+        return _not_found("design system or rulebook")
+    return jsonify(report)
+
+
 # ── Tokens ──────────────────────────────────────────────────────────────
 
 @design_systems_bp.get("/design-systems/<int:design_system_id>/tokens")