Grow compile-CI coverage to remaining green sketches + untracked boards

[jonfroehlich]

Follow-up to the START-GREEN policy in .github/workflows/compile-sketches.yml. CI currently gates only a verified-green subset; the workflow comments defer the rest with no issue behind them. This tracks closing that gap so coverage actually grows.

Remaining green sketches not yet gated

The workflow notes:

The remaining plain-Uno sketches in Sensors//PlayingMusic//AddressableLEDs//Serial/ all compile green and can be added to the Uno job in a follow-up.

Add these to the arduino:avr:uno matrix job. Note the constraint already documented in the workflow: arduino/compile-sketches compiles every sketch in a folder with that job's single FQBN (it does not read sketch.yaml), so mixed-board folders must list individual sketches under the matching board's job rather than the folder wholesale.

Boards with zero CI coverage

• UnoR4WiFi/ (arduino:renesas_uno:unor4wifi)
• CPX/ (adafruit:samd:adafruit_circuitplayground_m0)
• nRF52840/ — only two onboard-NeoPixel sketches are gated today; the rest of the folder is ungated

(ESP32/ is intentionally excluded here — it's tracked under #12, the core-v3.x migration.)

Done when

• Remaining green plain-Uno sketches added to the Uno job
• UnoR4WiFi/ and CPX/ get matrix jobs on their real FQBNs (or individual-sketch listings where folders are mixed-board)
• Any newly-included sketch that can't compile is either fixed or added to docs/legacy-sketches.md with a skip step