test/hil: erase MCU after tests; show throughput speeds in report

Teardown: instead of flashing device/board_test (a USB-less blink loop that
keeps the MCU busy-looping), erase the first flash sector (vector table) so the
board faults to idle after its tests — no USB, lower power, faster. Per-flasher
erase_<name>: openocd/openocd_adi `flash erase_sector 0 0 0`; stlink `--erase
0`; jlink erases the sector at the flash origin read from the ELF (pure-Python,
new elf_flash_origin); esptool `erase_region 0x0 0x4000`; lm4flash writes a 4 KB
all-0xFF blank image (lm4flash erases before programming, so the first sector
ends up blank). device/board_test flash remains a fallback for flashers with no
erase_ function. The teardown is no longer a report column (it's cleanup).

Report: cdc_msc_throughput and msc_file_explorer[_freertos] now return a compact
read/write speed shown in their report cell instead of the pass tick (e.g.
"C 652k/422k M 1.1M/783k", "rd 1.2MB/s"). test_example returns an optional
metric; render_matrix shows it verbatim. Firmware lookup factored into
find_firmware (reused by the erase teardown).

Verified on the rig (stm32f723disco, jlink): erase disables the board in 0.8 s
and it disappears from the bus; the throughput cell shows live speeds.

Co-Authored-By: Claude Opus 4.8 (1M context) <[email protected]>

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