Add backpressure to flash write protocol (#33)

* Add backpressure to handle_flash_write, use _write_block in write_flash

handle_flash_write now sends per-packet ACK (matching handle_write).
write_flash reuses _write_block for consistent backpressure behavior.

Flash write still fails at ~1MB due to FMC soft reset accumulation
during page programming. RAM write (16MB) verified at 921600 baud.
Flash write needs further debugging of fmc_enter_boot impact.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

* Fix agent CLI progress output, CRC32 timeout, and misleading labels

- Fix progress display: use plain print() instead of Rich console.print()
  which was adding newlines instead of overwriting in-place via \r
- Scale CRC32 verification timeout with data size (~1s/MB) to avoid
  timeout on large reads (16MB was timing out at 10s hard limit)
- Show "Sending Agent" instead of "Sending U-Boot" during agent upload
- Document flash agent commands in main README

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

* Make agent read/write default to flash base and auto-detect size

- `agent read` no longer requires -a and -s; defaults to 0x14000000 and
  queries flash size from the device via get_info()
- `agent write` defaults -a to 0x14000000
- `agent read` -o defaults to flash_dump.bin
- Update README examples to show the simpler invocation

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

* Fix mypy: cast flash_size to int for read_memory arg

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

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Co-authored-by: Dmitry Ilyin <widgetii@users.noreply.github.com>
Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: widgetii