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.

tests/canary/test_fingerprint_generators.py

@@ -101,3 +101,24 @@ def test_mint_uuid_stable_across_html_and_svg() -> None:
     html_uuid = next(n for n in html.notes if n.startswith("mint_uuid="))
     svg_uuid = next(n for n in svg.notes if n.startswith("mint_uuid="))
     assert html_uuid == svg_uuid
+
+
+def test_fingerprint_html_nonce_populated_and_matches_hmac() -> None:
+    """Artifact carries ``fingerprint_nonce`` matching HMAC derivation."""
+    import uuid as _uuid
+    from decnet.canary.obfuscator import nonce_for
+
+    art = get_generator("fingerprint_html").generate(_ctx("nonce-tok"))
+    assert art.fingerprint_nonce is not None
+    assert len(art.fingerprint_nonce) == 16
+    _MINT_NS = _uuid.UUID("a3f7c821-9d1e-4b6a-8c2d-1e4f9a7b3c5d")
+    expected_mint = str(_uuid.uuid5(_MINT_NS, "nonce-tok"))
+    expected_nonce = nonce_for("nonce-tok", expected_mint)
+    assert art.fingerprint_nonce == expected_nonce
+
+
+def test_fingerprint_svg_nonce_matches_html_for_same_token() -> None:
+    """Both generators derive the same nonce for the same callback token."""
+    html = get_generator("fingerprint_html").generate(_ctx("nonce-tok2"))
+    svg = get_generator("fingerprint_svg").generate(_ctx("nonce-tok2"))
+    assert html.fingerprint_nonce == svg.fingerprint_nonce