feat: advertise MCUmgr params from MCUboot serial recovery

[JPHutchins]

Closes #10.

Enables MCUboot's serial-recovery MCUmgr params command (mcu-tools/mcuboot#2746, merged 5fa7077) on the existing recovery fixtures so a client can auto-negotiate serial fragmentation against the bootloader's recovery server instead of being told a buffer size out of band.

No new fixture variants, and no Zephyr/west bump — the pinned Zephyr (f33aa2bc → mcuboot 0fae8920) already contains the merged command; this is a fixture-config change only.

What changed

• sysbuild/mcuboot_recovery.conf: CONFIG_BOOT_MGMT_MCUMGR_PARAMS=y, enabled once — covers every mps2 recovery fixture (do-it-all, raw, and the buffer matrix) via FILE_SUFFIX="recovery".
• sample.yaml: the build-only serial_recovery.nrf52840dk image gets the option as an explicit mcuboot_CONFIG_ extra_arg (it doesn't use the recovery suffix). Comment on the buffer matrix records that the sweep is app-side only.
• collect_fixtures.py: new manifest fields recovery_buf_size / recovery_buf_count, derived from the mcuboot image .config (CONFIG_BOOT_SERIAL_MAX_RECEIVE_SIZE and a constant 1), null unless a recovery build has the command enabled. The collector only read the app .config before, so the recovery server's advertised size wasn't surfaced.
• README: manifest field table + the serial_recovery row.

Design decisions (raised in #10)

• Buffer matrix: the bootloader's advertised recovery buf_size (BOOT_SERIAL_MAX_RECEIVE_SIZE, default 1024) is held constant across the matrix. The matrix sweeps the app netbuf only; the recovery server is a separate server and the encoded/raw negotiation math is already linear, so sweeping it would add bootloader RAM per row for no extra coverage.
• Manifest surfacing: added explicit recovery_buf_size / recovery_buf_count fields rather than documenting a constant, so the client test reads the advertised value from the manifest (SSOT) instead of hardcoding 1024. buf_count is a literal 1 in mcuboot's boot_serial.c (one command reassembled at a time), not a Kconfig.

Verification

Built both mps2 recovery fixtures locally (west twister -T apps -p mps2/an385, SDK 1.0.1):

• both mcuboot/zephyr/.config carry CONFIG_BOOT_MGMT_MCUMGR_PARAMS=y, CONFIG_BOOT_SERIAL_MAX_RECEIVE_SIZE=1024, CONFIG_BOOT_MAX_LINE_INPUT_LEN=128 (the command's dependency, intact even under BOOT_SERIAL_RAW_PROTOCOL=y);
• running the collector over the build surfaces recovery_buf_size=1024, recovery_buf_count=1 for both encoded (serial) and raw (serial_raw) recovery, distinct from the app netbuf.

ruff check / ruff format --check / mypy --strict / pytest (incl. doctests) all green.

Enables

intercreate/smpclient#120 — integration coverage for the recovery negotiation path (re-vendors these fixtures once a release is cut).

🤖 Generated with Claude Code

https://claude.ai/code/session_01KKXy7L2Udjw929nie2c363

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