ClearSession does not abort Bitcoin signing or fully scrub signer state

[BitHighlander]

Summary

ClearSession does not abort an in-flight Bitcoin signing state machine, and the branch's signing_abort() does not fully scrub the signing key material it does own.

Found while auditing PR #604 at exact head 1d446ccbc9cf32a6e499990e65bcf7b358f1325b.

Evidence

• lib/firmware/messagemap.def:15-20 registers SignTx, TxAck, and ClearSession as independently dispatchable inbound messages.
• lib/board/messages.c:122-139 dispatches any mapped message directly; it has no expected-message-type gate for an active signing session.
• lib/firmware/fsm.c:300-303 handles ClearSession by calling only session_clear(true) and returning success.
• By contrast Initialize and Cancel call signing_abort() (fsm_msg_common.h:1-10, :582-590).
• lib/firmware/signing.c:96 holds the current raw private key in file-static privkey[32], populated at :1857.
• signing_abort() at :2151-2159 wipes node and the root pointer itself, but not privkey, transaction/hash state, or the HDNode referenced by root.

Closed issues #452/#546 record the private-key/root fixes on alpha; those fixes are absent from this 7.14.3 head.

Impact

High — authorization loss is not a signing boundary. After transaction approval and before the final streamed TxAck, a host can clear the session and continue the still-live signer. Separately, abort leaves a spend key resident for the boot lifetime.

Acceptance criteria

• Make ClearSession, lock, wipe, Initialize, and Cancel invoke one central Bitcoin signing teardown before reporting success.
• Backport the closed alpha private-key/root scrubbing fixes and wipe all signing accumulators/state that can carry authorization or key material.
• Add an integration test that interposes ClearSession at every TxRequest stage and proves every later TxAck is rejected with no signature/serialization.
• Add unit instrumentation proving confidential signing buffers are zero after success and every abort path.

[BitHighlander]

Resolved on PR #604 at exact reviewed head d842f1e. ClearSession now reaches centralized Bitcoin signing abort and scrubs signer statics, private/root/derived nodes, and digests. Wire coverage interrupts every TxRequest stage; both exact-head CI gates passed. Re-review attestation: #604 (comment). Leaving this issue open until merge.