Security audit fixes for B13

[3rdIteration]

Fixes findings 1–6 from the B13 release security audit. To be merged as part of B13.

Fixes

1. Verify Signature passed unsigned lines in clearsigned manifests (High). gpg --verify reports VALIDSIG even when unsigned text is added before or after a clearsigned block, or a second block is appended. Confirmed on GnuPG 2.4.9. Check SHA256Sum then hashed every line of the raw file, so an added <hash> <file> line showed as a green "Matched SHA256". Clearsigned files with any content outside the signed block are now refused. Manifests with an embedded signature are hash-checked only from the text gpg --decrypt reports as signed, never from the raw file. Detached .sig/.asc behaviour is unchanged.
2. Provision MicroSD could write outside the card (Medium). sd_update.txt is unsigned, and the image names it lists were joined onto the card root, so a name like ../x copied a file (as root) outside it. Only bare filenames are now accepted. Bad names are refused before the write-length fix or any copy happens.
3. Secrets in logs (Medium). encryption_key (the EncryptedQR/KEF passphrase) and text (typed Text QR content) are now redacted from Destination reprs, which are logged at INFO.
4. Release-signing keys survived the wipe (Medium). SeedKeeper-loaded keys left behind by an abandoned flow were never cleared by Home or the inactivity wipe. Both now clear those keys and the BIP85 cache via clear_signing_keys().
5. Blind digest signing (Medium). Sign Digests and air-gap Rounds 1–2 now show a "Signing blind" confirmation before any key is loaded. Back cancels.
6. Subkey signers marked untrusted (Low). _parse_gpg_verify_status now takes the primary-key fingerprint from the last field of VALIDSIG. Releases signed with a subkey now match the trusted-signer list, which holds primary-key fingerprints.

Tests

• New tests for each fix. All 14 fail on dev and pass here.
• Air-gap ceremony flow test: its signing rounds now press "Sign" on the new confirmation.
• Local full suite, excluding live-hardware tests: 2564 passed. The three remaining failures are environment-only and also fail on dev: two need compiled .mo translations that aren't built locally; one is a local pysatochip install older than the pinned build.

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