feat(gui): Add an optional testkey bootloader step to Firmware Flash

[miner7222]

Why

Installing testkey-signed firmware on a device whose bootloader trusts the Lenovo key previously meant downgrading the entire firmware first, just to get back to a bootloader that would accept the re-signed chain. That is slow, risky, and throws away the newer build the user actually wanted.

This lets the user hand LTBox a bootloader that still trusts the existing key. The run then re-signs the required partitions and flashes the supplied abl.elf at the end, so the device boots the new firmware without a full-firmware downgrade.

What changed

Firmware identity moves to the folder step. Pressing Next on the firmware folder now inspects vbmeta_system.img once, reports the signing key and the model from the build fingerprint in a dialog, and caches the verdict. The flash worker consumes that cached result instead of classifying the same image a second time. A folder whose vbmeta_system.img cannot be read blocks Next rather than failing later mid-run.

New optional bootloader step. It appears only when the firmware is Lenovo-key signed and the fingerprint is not TB323FU / TB376FC / TB390FU. The step indicator grows to seven entries only in that case. Picking an .elf extracts the AVB public key embedded in the ABL and labels it Test key / Lenovo key / Unknown; Next unlocks only for a test key, or when nothing is selected at all. Skipping the step leaves the run byte-for-byte identical to today.

Worker. With a user-supplied ABL the run takes the testkey re-sign path unconditionally and no longer dumps the device's own bootloader. It still reads device vbmeta_system, because the rollback-index floors from that dump are what keep the re-signed images from being rejected. The supplied abl.elf is written to abl_a (LUN 4) last, after the rawprogram flash and the ARB overlays, in place of the device-ABL restore.

ltbox_patch::abl_key. A pure-Rust port of the reference extract_avbpublickey.py: LZMA-alone decode, PE scan, AVB key-blob validation (n0inv + Montgomery rr), AArch64 ADR/ADRP+ADD reference counting to pick the referenced key, then SHA-1 over the blob. Classification reuses the existing key_map. The only dependency change promotes lzma-rust2, already present transitively, to a direct dependency — no new crate enters the graph.

Verification

• Verified on hardware.
• cargo fmt --check, cargo clippy --workspace --all-targets -D warnings, cargo test --workspace all clean (the pre-push hook runs these).
• New unit tests cover the visible step sequence in both configurations and the gate decision across key classes and gated models.
• The ABL extractor reproduces the reference script's output on seven local ABL images (six test key, one Lenovo key) under LTBOX_TEST_ABL_DIR.

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