Cold wallet: review the call a multisig execute dispatches

[n13]

Stacked on #622. Review that one first. This PR is the cold wallet display, the reference parser, and a QR test corpus.

Showing the executed call

With chain#675, multisig.execute carries the call it dispatches, and the chain only dispatches it if it matches the stored proposal. So the cold wallet can show that call instead of a proposal id under a note saying its contents are not signed.

CallDecoder now describes execute the way it already describes approve: a nested "You are executing" call that also supplies the summary, so the screen headlines SEND with the amount and recipient. cancel and remove_expired keep the old note, which is still true for them.

Reference parser

rust-transaction-parser had drifted from the chain in two ways.

approve was already broken. The chain added the bound call bytes to approve some time ago, and this parser still decoded the old two-field shape. It could not parse a current approval at all. It now decodes and shows the approved call.

execute gains the inner call. Because it is inline rather than length prefixed, SCALE decoding recurses before the parser's own depth check runs. About 200 nested executes fit in the payload cap. Decoding now uses decode_with_depth_limit, so that fails on a counter instead of the stack. The three duplicated depth checks are now one helper.

Both parsers also cap nested calls at 10 KiB, the chain's own MaxCallSize, and raise the payload cap to 12 KiB so a chain-maximum proposal can arrive.

QR test corpus

New: cold-wallet-app/test/fixtures/qr/reduced/. Nine signing requests, one per call the Keystone firmware parses today.

Case	Payload	QR frames
transfer_allow_death	118 B	2
transfer_keep_alive	119 B	2
schedule_transfer	128 B	2
schedule_transfer_with_delay	137 B	2
create_multisig, 2 of 3	188 B	3
propose wrapping a transfer	158 B	3
approve wrapping a transfer	158 B	3
execute wrapping a transfer	157 B	2
execute wrapping a reversible transfer	166 B	3

Each case has the payload hex, the envelope JSON, the UR parts, one SVG per QR frame, and an index.html that animates them. There is a top-level index.html to pick a case. Open it in a browser and show it to a device camera or the simulator, which scans by screenshotting the screen.

A test walks the manifest and parses every case. It checks each decodes to the call it claims, including the nested call for propose, approve and execute. It also checks the manifest matches the folders on disk, and that the corpus targets the runtime the app bundles. So regenerating metadata and forgetting the corpus fails the build instead of going unnoticed.

I also ran all nine through the firmware Rust parser. All nine decode there too.

The corpus is generated, not hand written. See the quantus-cli generate_qr_fixtures example, added in quantus-cli#145. It encodes calls through the CLI's bundled chain metadata, so the bytes match what the chain accepts. The README next to the fixtures has the command.

Why this is the reduced set

The payload is hex inside JSON, so QR data is about twice the payload size, and UR splits it at 200 bytes. Even a 118 byte transfer needs 2 frames. A chain-maximum 10 KiB call would be roughly 100 frames, which is not practical to scan. A fuller set can be added later, but big calls should be treated as a deliberate stress case, not a normal one.

set_code is deliberately absent. It is being replaced by approving a hash for runtime upgrades.

Tests

quantus_sdk 264, cold-wallet-app 60, rust-transaction-parser 24.

[n13]

Superseded. This branch is stacked on #622, which is based on main before the 1.5.12+129 snapshot (78a698f) — that snapshot rewrote call_decoder.dart, added call_policy.dart, regenerated part of the bindings and deleted rust-transaction-parser/, so both branches conflict heavily. Redoing the work as a single PR on current main. Branch deleted; restorable from this page if the diff is needed as reference.