Summary

• pass OTA chunks to RpcClient.send as scalar strings so its single-parameter wrapper emits the firmware payload shape expected by the ESP32-C6 fixture
• enlarge the ESP32 AutoResearch serial RX queue before Serial.begin() so base64 JSON-RPC OTA frames fit
• keep RP2350W HTTP responses length-delimited and let the peer close first, with a bounded fallback, so repeated peer cycles do not exhaust lwIP TCP PCBs
• delegate explicitly named absent RP2040/RP2350 runtime ports to fbuild's CDC/BOOTSEL recovery ladder while retaining the early failure for non-RP ports
• add focused regression contracts for OTA argument shape, serial buffering, passive-close behavior, and the RP/non-RP port-preflight boundary

Validation

• uv run --no-sync ty check ci/autoresearch/ota.py ci/rpc_client.py
• focused AutoResearch Python regressions (RED before implementation, GREEN after)
• uv run --no-sync pytest -q ci/tests/test_autoresearch_phases.py ci/tests/test_usb_power.py — 204 passed
• uv run --no-sync ruff check ci/autoresearch/phases.py ci/tests/test_autoresearch_phases.py
• broader selected AutoResearch Python regression set
• bash test --cpp — 284 unit tests and 84 host examples passed
• bash lint --cpp
• bash compile rp2350w --examples AutoResearch
• bash compile esp32c6 --examples AutoResearch
• pre-push review clean

Dependency and merge order

Depends on FastLED/fbuild#1405.

1. Merge fix(rp): reacquire same-name CDC after BOOTSEL deploy fbuild#1405.
2. Release the first fbuild version newer than 2.5.21 containing that fix.
3. Update this branch's exact fbuild pin and uv.lock to that release.
4. Complete the attached RP2350W + ESP32-C6 network and OTA HIL runs, then merge this PR.

Released fbuild 2.5.21 can copy an RP2350 UF2 image but may omit FBUILD_DEPLOY_PORT when Windows changes a stale COM18 devnode back to healthy under the same name. #1405 makes the pre-flash snapshot health-aware so both explicit UF2=<volume> and stale direct-COM selectors converge on the recovered healthy CDC endpoint.

Hardware gate still pending

The exact RP2350W is serial 2DCB876B587EA334 (runtime COM18 / BOOTSEL H:); the ESP32-C6 is serial 8C:BF:EA:CF:87:B4 on COM9. The released-fbuild run reproduced the same-name CDC handoff failure after a successful UF2 copy. Final canonical network and OTA assertions will be recorded here after the dependent release is pinned.

Closes #4059
Refs #3956
Refs #3832