feat: user-provisioned metadata signing keys — slots 4-6 + storage v18

[BitHighlander]

Summary

Lets KeepKey owners add their own secp256k1 signing keys at runtime so companies / DAOs can publish their own clear-signing descriptor sets that members verify on their own device, without a firmware update.

• 3 user slots (4..6) persisted in Storage.pub.metadata_keys
• STORAGE_VERSION 17 → 18 with explicit V17→V18 migration that zero-inits the new key array on existing devices
• AddMetadataKey / RemoveMetadataKey / ListMetadataKeys handlers in fsm_msg_metadata.h
  • Add: 2-step confirm (slot+label, then SHA-256 fingerprint), then PIN
  • Remove: confirm shows fingerprint of the key being removed (defends against stealth removal on a briefly-unlocked device)
  • Slots 0-3 remain firmware-baked and cannot be overwritten
• signed_metadata.c moved from a hardcoded METADATA_PUBKEYS lookup to a unified metadata_get_pubkey() that resolves slot 0-3 from the static firmware table and slot 4-6 from storage
• Verification confirm screen now shows the matching label: "Insight Verified" for slot 0, "<label> Verified" for user slots

Depends on

BitHighlander/device-protocol#26 — pulls submodule pin to that branch's tip.

Trust model

Slot	Source	Mutable?
0	Firmware-baked: "Insight" (KeepKey-managed)	No
1	Firmware rotation slot	No
2	Firmware reserved	No
3	CI test (DEBUG_LINK only)	No
4-6	User-provisioned via PIN+confirm	Yes (replace/remove)

Add/remove require PIN entry + 2 confirms (1 for label/slot, 1 for fingerprint). The fingerprint is SHA-256 of the pubkey, first 4 bytes (8 hex chars) — small enough for the OLED, stable enough to verify against a company-published value out-of-band.

Storage layout

V18 uses the V17 reserved region at offset 501 in the plaintext block:

• 4 bytes: metadata_keys_count (u32 LE)
• 3 × 51 bytes: { used(1) + pubkey(33) + label(17 nul-padded) }
• Total: 157 bytes within the 996-byte V17 reserved range

V17 storage on disk reads cleanly — storage_fromFlash zeros the new fields explicitly on the V17 case (memzero(dst) at entry already does it; explicit reset documents the invariant).

Test plan

• CI builds clean (proto regen, firmware ARM build, native emulator)
• V17 → V18 storage migration: existing-device flash dump loads correctly with empty user-key set
• AddMetadataKey: confirm screens display label and fingerprint; PIN required
• RemoveMetadataKey: fingerprint confirm shown for the key being removed
• ListMetadataKeys: returns only populated user slots (0-3 firmware slots intentionally not enumerated)
• signed_metadata flow with user-slot key: ECDSA verify succeeds, confirm shows "<label> Verified"
• Reject: oversize label (>16 chars), uncompressed pubkey prefix, slot out of [4,6]
• Wipe device: user keys cleared along with rest of storage

Future / not in this PR

• Bulk-signing build for an "Insight signing device" (custom firmware that batch-signs metadata payloads without per-payload button confirm)
• Listing firmware-baked slots 0-3 in MetadataKeysList for full visibility
• Unit tests in unittests/firmware/signed_metadata.cpp (need the build toolchain re-installed locally — leaving for follow-up)

[BitHighlander]

Pre-existing CI note

include/keepkey/emulator/setup.h and include/keepkey/emulator/libkkemu.h show clang-format failures, but these files are identical to alpha (git diff alpha..feat/user-metadata-keys for them is empty) and alpha's own CI run on commit 92c8f534 (the libkkemu fix-up) failed lint for the same reason.

The format issues there are pre-existing and not introduced by this PR. Either fixing them up here as a one-line follow-on or in a separate alpha-cleanup PR is fine — happy to do whichever you prefer.