fix(security): fault injection hardening, nanopb memleak, BIP39 recovery validation

[BitHighlander]

Motivation

Three independent security improvements bundled together as they're all defensive hardening with no feature dependencies between them.

Changes

1. Fault injection hardening — signatures_ok() (F3)

The existing signatures_ok() validates firmware signatures by checking each individually and returning on first failure. This is vulnerable to single-glitch fault injection where an attacker skips the failure branch.

Fix: Infective aggregation pattern — accumulate all signature results into a single variable using XOR, then double-hash verify the aggregate. A single glitch cannot bypass all checks because the final comparison depends on every intermediate result.

2. Metadata-based signature check (F5)

The firmware-side signatures_ok() duplicates logic already performed by the bootloader. An attacker who glitches past the bootloader check faces the same vulnerable pattern in firmware.

Fix: Replace the firmware-side cryptographic check with a metadata flag set by the bootloader. The bootloader's check is the authoritative one (runs in a more constrained environment); firmware trusts it via a tamper-evident metadata field rather than re-implementing the same vulnerable check.

3. Nanopb oneof memory leak (backport of upstream 4fe23595)

Nanopb's pb_release() doesn't properly free memory allocated for oneof fields when the active field changes. Over repeated message decode cycles, this leaks memory on the embedded device.

Fix: Backport the upstream nanopb fix that tracks and frees the previous oneof allocation before assigning a new one.

4. BIP39 wordlist validation during cipher recovery

The cipher recovery flow accepts any character sequence without validating against the BIP39 wordlist. A user entering an invalid word only discovers the error after completing all 12/24 words, forcing a full restart.

Fix: Add per-word validation after each word is completed during cipher recovery. If the decoded word doesn't match any BIP39 entry, reject immediately with Failure_SyntaxError so the user can correct it in place.

Risk assessment

• F3/F5: Modifies lib/board/signatures.c — critical path but the change is strictly more defensive. The existing tests exercise signature validation.
• Nanopb: Single-file change in pb_decode.c, isolated to memory management.
• BIP39: Modifies recovery_cipher.c — user-facing but fail-safe (rejects invalid input rather than accepting it).

Testing

• All existing unit tests pass
• Python integration tests pass
• ARM cross-compile clean