Pad U-Boot to boot-partition size on install (#73)

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Summary

• OpenIPC removed the historical 1 MiB padding from published U-Boot assets (issue Pad u-boot-{hi3516av200,hi3519v101}-universal.bin to flash erase-block size before writing #73). For example u-boot-hi3516av200-universal.bin went from 1,048,576 B (1 MiB padded) → 199,276 B (raw).
• Add pad_to_size() in firmware.py and call it at the install read site so the U-Boot region is written with 0xFF trailing bytes — matches the historical 1 MiB on NAND and 256 KiB on NOR (clean partition boundaries either way).
• Agent flash path (LZMA SPL boundary detection) is unaffected. Restore path doesn't write U-Boot to flash, only loads it to RAM via the bootrom — neither needs to change.

Closes #73

Test plan

• uv run pytest tests/test_firmware.py -x -v — 16/16 pass (5 new TestPadToSize cases)
• uv run pytest tests/ -x -q --ignore=tests/fuzz — 407 passed, 2 skipped
• uv run ruff check src/defib/firmware.py src/defib/cli/app.py tests/test_firmware.py — clean
• uv run mypy src/defib/firmware.py src/defib/cli/app.py --ignore-missing-imports — clean
• make -C agent test HOST_CC=gcc — 5406/5406
• After OpenIPC ships unpadded asset: re-run defib install -c hi3516av200 … on av200 hardware and confirm the boot partition still flashes clean.

🤖 Generated with Claude Code

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Hardware verification — av200 board (hi3516av200, NAND)

Tested with the cached u-boot truncated to 199,276 B to simulate the new (unpadded) OpenIPC producer asset:

$ ls -l ~/.cache/defib/firmware/u-boot-hi3516av200-universal.bin
-rw-r--r-- 1 root root 199276 ...

defib burn (full bootrom upload pipeline, smallest input):

SPL boundary detected (LZMA) at 0x6800 (26624 bytes); profile default was 0x4F00 (20224 bytes)
  Sending DDR step  100%
  Sending SPL       100%   ~2s
  Sending U-Boot    100%  ~18s
Done! (27927ms)

defib agent upload (uses same truncated u-boot as SPL):

Agent: agent-hi3519v101.bin (15812 bytes)
SPL: full U-Boot (199276 bytes — boundary auto-detected)
  Boot mode entered → DDR step → SPL → Agent → READY
Agent ready!
  RAM: 0x80000000
  Flash: 131072KB

defib agent info:

JEDEC ID:    00c212    (Macronix MX35LF1GE4AB SPI NAND)
Flash size:  131072 KB
RAM base:    0x80000000
Sector size: 128 KB
Agent ver:   2
Capabilities: flash_stream, sector_bitmap, page_skip, set_baud, reboot, selfupdate, scan

The bootrom upload + agent stages handle the new (smaller) producer asset cleanly. pad_to_size itself is unit-tested; only the tftp_and_flash step in install exercises it on hardware, but that step writes a fixed-size buffer to a fixed-size partition — the only behavioral difference vs. the old producer is 0xFF vs. 0x00 trailing bytes, and 0xFF is the natural NAND erased state, so it's actually more correct than before.