sec(license): Ed25519 sigs + production-safe tripwire

Two coupled hardening upgrades.

1. Asymmetric signatures (HMAC → Ed25519)

The previous HMAC scheme used a symmetric secret that any motivated
reverse engineer could pull out of the shipped binary and use to
mint blobs for any tier / name / email. With Ed25519, the binary
ships only the public verification key; the signing key never
leaves the seller's environment, so binary compromise no longer
yields forgery.

- src/license/crypto.py rewritten around
  cryptography.hazmat.primitives.asymmetric.ed25519. Same public
  API surface (sign/verify/encode_blob/decode_blob), same canonical
  JSON encoding — drop-in for the manager / cli / GUI layers.
- DATATOOLS_LICENSE_PRIVKEY (seller-side) and
  DATATOOLS_LICENSE_PUBKEY (build-time) env vars supply the keys;
  the in-source dev keypair (src/license/_dev_keypair.py)
  deterministically derives from a seed phrase for repro builds and
  tests.
- Blob prefix bumped DTLIC1: → DTLIC2:. Decoding a DTLIC1 blob
  surfaces a clear "old format" error rather than a confusing
  signature mismatch.
- scripts/generate_keypair.py mints fresh production keypairs for
  the seller (run once, stash the private key offline). Adds
  cryptography>=41,<46 to requirements.txt (was an undeclared
  transitive dep).

2. Production-safe tripwire

assert_production_safe() refuses to boot a frozen / shipped build
when either:

- DATATOOLS_DEV_MODE=1 is set (would unconditionally bypass every
  license check — fine in source/test but catastrophic in a buyer
  install).
- The active verification key is still the embedded dev key (the
  build pipeline forgot to set DATATOOLS_LICENSE_PUBKEY).

No-op in source / pytest runs (sys.frozen is unset) so test
fixtures and dev workflows keep working without ceremony. Called
from src/cli_license_guard.guard() and from hide_streamlit_chrome
— so it fires on every CLI invocation and every GUI page load.

Tests: 49 license-layer unit tests (was 40); added Ed25519
wrong-key rejection, dev-keypair seed pin, blob v2 prefix, v1
rejection with clear message, and four production-safe scenarios
(no-op in source, fires on DEV_MODE in frozen, fires on dev key in
frozen, passes in frozen with prod pubkey). Total: 2024 → 2033.

Docs (REQUIREMENTS §17a, DEVELOPER licensing recipe, DECISIONS
§9b + decision log) updated with the new threat-model write-up,
key-storage workflow, and tripwire behaviour.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/gui/components/_legacy.py

@@ -89,6 +89,12 @@ def hide_streamlit_chrome(*, gate_license: bool = True) -> None:
     can render its own form without recursion.
     """
     st.markdown(_HIDE_CHROME_CSS, unsafe_allow_html=True)
+    # Production-safe check runs first so a misconfigured shipped
+    # build refuses to render anything (rather than rendering a
+    # broken activation form that doesn't accept real blobs).
+    # No-op in source / pytest runs.
+    from src.license import assert_production_safe
+    assert_production_safe()
     # Imported lazily so this module stays importable in environments
     # where the i18n packs haven't been laid out (e.g. unit tests of
     # individual legacy helpers).