fix(catalog-console): close the vacuous review gate; split catalog/release signing keys

Fixes #68 findings 1 and 5.

Finding 1 -- the review gate signed without reviewing anything:
- ReviewWindow._on_sign hardcoded "main" as the TOCTOU comparison ref, so
  any PR review (where _on_load pins the PR head's blob SHA) could never
  sign; the only working path was main-vs-itself, whose empty diff made
  can_sign() vacuously True (set() <= set()). Commit b08cf21 signed 19
  entries through exactly that path with zero of them reviewed.
- can_sign() now refuses an empty changeset outright, and itself checks
  has_blocking_risk() across every changed entry rather than trusting the
  GUI to have disabled a checkbox.
- ReviewSession now carries loaded_ref (the exact ref reviewed); a new pure
  sign_precondition(session, resolve_blob_sha) resolves the TOCTOU SHA from
  that ref, never a hardcoded "main". _on_load's retry path re-diffs
  instead of re-pinning the same stale SHA, so a blob-mismatch refusal
  can't loop forever.
- source="main" now diffs against the last catalog a maintainer actually
  SIGNED (walking catalog.json's git history until a version verifies
  against the current .sig), not against itself.
- Replaced the theatre-only test_no_acknowledge_all_function_exists (only
  asserted no function was *named* acknowledge_all) with a test that also
  exercises the real gate. Added can_sign/sign_precondition coverage for
  the empty-diff, blocking-risk, and ref-resolution seams -- each verified
  to fail when its guard is removed.

Finding 5 -- the catalog key and release key were the same CI-resident key:
- scripts/sign_checksums.py gets its own RELEASE_PUBKEYS (separate from
  bcc_core.CATALOG_PUBKEYS) and a verify_checksums_against_any() helper.
- release.yml's signing-smoke-test now verifies RELEASE_SIGNING_KEY against
  RELEASE_PUBKEYS only -- it no longer imports bcc_core/CATALOG_PUBKEYS at
  all, so this workflow can never compare a CI secret against the
  catalog's root of trust.
- catalog_console.py: keygen/show-seed-b64 gain --release, with separate
  keychain/file storage per key kind. show-seed-b64 refuses to run without
  --release, so the catalog seed can't be exported to a CI secret by habit.
- README documents both keys' trust properties and the asymmetry: a CI
  compromise burns the release key, never the catalog key.

The maintainer must rotate the catalog key (it was CI-resident, so treat it
as burned for catalog use) and generate a fresh release key -- see the PR
description for the exact steps. No key is generated or committed here.

scripts/sign_checksums.py

@@ -42,6 +42,25 @@ from pathlib import Path
 # message signed by the same key.
 DOMAIN_PREFIX = b"bcc-release-v1|"
 
+# Public half of the RELEASE signing key(s) -- a SEPARATE keypair from
+# bcc_core.CATALOG_PUBKEYS (issue #68 finding 5). The catalog key is the
+# offline, Console-only root of trust for what BCC executes; this key is
+# CI-resident and signs ONLY the release SHA256SUMS manifest, never the
+# catalog. Keeping them apart means a CI/repo-secret compromise burns the
+# release key -- annoying, but it never lets an attacker sign a catalog a
+# user's binary would trust. A LIST (not a single key), mirroring
+# CATALOG_PUBKEYS, so the release key can be rotated without invalidating
+# the signature on every past release: verification accepts a match against
+# ANY key here.
+#
+# Empty until the maintainer generates the release keypair (separately from
+# the catalog keypair) and pastes the public half in:
+#     python catalog_console.py keygen --release
+# This is intentionally NOT pre-populated with a placeholder that looks
+# like a real key -- release.yml's signing-smoke-test fails closed (loudly)
+# on an empty list rather than silently verifying against nothing.
+RELEASE_PUBKEYS: list[bytes] = []
+
 CHUNK_SIZE = 1024 * 1024
 
 
@@ -135,6 +154,19 @@ def public_key_b64_from_seed(seed_b64: str) -> str:
     return base64.b64encode(raw).decode("ascii")
 
 
+def verify_checksums_against_any(pubkeys: list[bytes], sums_text: str, signature: bytes) -> bool:
+    """Verify `signature` against ANY key in `pubkeys` (each a raw 32-byte
+    Ed25519 public key). Mirrors bcc_core.verify_catalog_signature's
+    rotation-friendly "any currently-trusted key" semantics, applied to
+    RELEASE_PUBKEYS instead of the catalog's key list. Returns False (never
+    raises) for an empty `pubkeys` list -- fails closed rather than
+    vacuously verifying against nothing."""
+    return any(
+        verify_checksums(base64.b64encode(pk).decode("ascii"), sums_text, signature)
+        for pk in pubkeys
+    )
+
+
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