feat(canary): API-trashing defense — 4-layer fingerprint validation

Adds per-mint nonce gating, structural shape validation, mint UUID
consistency checks, and a per-(token, IP) rate limiter to the canary
worker so attackers who extract a canary from a decky filesystem cannot
poison fingerprint forensics by replaying or forging ?d= submissions.

Changes:

base.py
  fingerprint_nonce: Optional[str] added to CanaryArtifact so generators
  can surface the nonce to the cultivator without coupling the generator
  directly to DB code.

obfuscator.py
  nonce_for(callback_token, mint_uuid): HMAC-SHA256 keyed on
  DECNET_CANARY_FINGERPRINT_SECRET, truncated to 16 hex chars.
  FingerprintSecretMissing raised at mint time if env var is unset.
  render_fingerprint_js() now accepts nonce= and substitutes MINT_NONCE.

fingerprint_payload.js
  New MINT_NONCE placeholder. Appended as &k= on all beacon URLs (bare-open,
  single-shot, chunked). Using &k= avoids colliding with &n= (chunk total).

fingerprint_html.py / fingerprint_svg.py
  Derive nonce via nonce_for() and pass to render_fingerprint_js(). Set
  artifact.fingerprint_nonce so the cultivator can persist it.

cultivator.py
  Passes fingerprint_nonce into create_canary_token() when present on the
  artifact; NULL for all non-fingerprint generators.

canary.py (model)
  fingerprint_nonce: Optional[str] = Field(default=None, max_length=16)
  added to CanaryToken. None for non-fingerprint tokens.

worker.py
  _extract_fingerprint now returns (meta_dict, parsed_fp) tuple.
  _record_hit accepts parsed_fp + raw_nonce and runs 4 layers after
  token lookup: nonce match, shape check, mint UUID consistency, rate limit.
  Each failure sets _fp_invalid_* flag and drops structured _fp.
  Trigger row always lands regardless.

tests/canary/conftest.py
  Session-scoped autouse fixture sets DECNET_CANARY_FINGERPRINT_SECRET so
  fingerprint generator and worker tests work offline.

tests
  5 new worker HTTP tests and 2 new generator tests covering each
  validation layer.

decnet/canary/cultivator.py

@@ -160,7 +160,7 @@ async def cultivate(
     # attribute a callback if the artifact trips during the plant
     # itself (improbable but possible — DOCX viewers can preview
     # autoplay-style).
-    await repo.create_canary_token({
+    token_data: dict = {
         "kind": _GENERATOR_TO_KIND.get(gen_name, "http"),
         "decky_name": plan.decky_name,
         "instrumenter": None,
@@ -171,7 +171,10 @@ async def cultivate(
         "placed_at": datetime.now(timezone.utc),
         "created_by": created_by,
         "state": "planted",
-    })
+    }
+    if artifact.fingerprint_nonce is not None:
+        token_data["fingerprint_nonce"] = artifact.fingerprint_nonce
+    await repo.create_canary_token(token_data)
 
     # Carry the placement_path on the artifact so the orchestrator's
     # plant_file call uses it.  We don't mutate the generator's