CI: compile examples on AVR, ESP32, SAMD and RP2040

[NT7S]

Adds an arduino/compile-sketches matrix that builds all examples for uno, esp32, mkrzero and rpipico. It reports sketch sizes and posts size-delta comments on PRs. The existing arduino-lint (v2) job stays.

• si5351_vcxo is left out of the build for now, because it doesn't compile on master (get_correction() is missing an argument). Add I2C bus selection (TwoWire*) constructor; fix si5351_vcxo example #107 fixes that, so add it back after Add I2C bus selection (TwoWire*) constructor; fix si5351_vcxo example #107 merges.
• Host tests: only a placeholder comment for now. Test Lab will add the test job in a separate PR.
• This only changes CI, so the library version stays the same.
• actionlint passes.

Independent review (b8e040b): PASS. A fresh reviewer found nothing blocking. All CI jobs pass: lint, uno, esp32, mkrzero, rpipico, and the size-delta report. There are some optional follow-ups:

• Size comments won't post on PRs from forks.
• Add a top-level permissions: contents: read.
• Restrict push to master so each PR doesn't run CI twice.
• Give each matrix job a clearer name.

Cleanups (9d1685e): jobs now get a read-only token by default, push builds run only on master, each job is named for its board, and size-delta comments come from a scheduled size-deltas.yml so they work on fork PRs. Independent review: PASS, with nothing blocking. CI passes on all jobs. Size comments only start after this merges, because scheduled workflows run from the default branch only. The si5351_vcxo example should be re-added after #107 merges.

[github-actions]

Memory usage change @ b8e040b

Board	flash	%	RAM for global variables	%
arduino:avr:uno	0 - 0	0.0 - 0.0	0 - 0	0.0 - 0.0
arduino:samd:mkrzero	0 - 0	0.0 - 0.0	0 - 0	0.0 - 0.0
esp32:esp32:esp32	0 - 0	0.0 - 0.0	0 - 0	0.0 - 0.0
rp2040:rp2040:rpipico	0 - 0	0.0 - 0.0	0 - 0	0.0 - 0.0

Click for full report table

Board	examples/si5351_calibration flash	%	examples/si5351_calibration RAM for global variables	%	examples/si5351_clk67_example flash	%	examples/si5351_clk67_example RAM for global variables	%	examples/si5351_example flash	%	examples/si5351_example RAM for global variables	%	examples/si5351_ext_ref flash	%	examples/si5351_ext_ref RAM for global variables	%	examples/si5351_outputs flash	%	examples/si5351_outputs RAM for global variables	%	examples/si5351_phase flash	%	examples/si5351_phase RAM for global variables	%	examples/si5351_sweeper flash	%	examples/si5351_sweeper RAM for global variables	%
arduino:avr:uno	0	0.0	0	0.0	0	0.0	0	0.0	0	0.0	0	0.0	0	0.0	0	0.0	0	0.0	0	0.0	0	0.0	0	0.0	0	0.0	0	0.0
arduino:samd:mkrzero	0	0.0	0	0.0	0	0.0	0	0.0	0	0.0	0	0.0	0	0.0	0	0.0	0	0.0	0	0.0	0	0.0	0	0.0	0	0.0	0	0.0
esp32:esp32:esp32	0	0.0	0	0.0	0	0.0	0	0.0	0	0.0	0	0.0	0	0.0	0	0.0	0	0.0	0	0.0	0	0.0	0	0.0	0	0.0	0	0.0
rp2040:rp2040:rpipico	0	0.0	0	0.0	0	0.0	0	0.0	0	0.0	0	0.0	0	0.0	0	0.0	0	0.0	0	0.0	0	0.0	0	0.0	0	0.0	0	0.0

Click for full report CSV

Board,examples/si5351_calibration<br>flash,%,examples/si5351_calibration<br>RAM for global variables,%,examples/si5351_clk67_example<br>flash,%,examples/si5351_clk67_example<br>RAM for global variables,%,examples/si5351_example<br>flash,%,examples/si5351_example<br>RAM for global variables,%,examples/si5351_ext_ref<br>flash,%,examples/si5351_ext_ref<br>RAM for global variables,%,examples/si5351_outputs<br>flash,%,examples/si5351_outputs<br>RAM for global variables,%,examples/si5351_phase<br>flash,%,examples/si5351_phase<br>RAM for global variables,%,examples/si5351_sweeper<br>flash,%,examples/si5351_sweeper<br>RAM for global variables,%
arduino:avr:uno,0,0.0,0,0.0,0,0.0,0,0.0,0,0.0,0,0.0,0,0.0,0,0.0,0,0.0,0,0.0,0,0.0,0,0.0,0,0.0,0,0.0
arduino:samd:mkrzero,0,0.0,0,0.0,0,0.0,0,0.0,0,0.0,0,0.0,0,0.0,0,0.0,0,0.0,0,0.0,0,0.0,0,0.0,0,0.0,0,0.0
esp32:esp32:esp32,0,0.0,0,0.0,0,0.0,0,0.0,0,0.0,0,0.0,0,0.0,0,0.0,0,0.0,0,0.0,0,0.0,0,0.0,0,0.0,0,0.0
rp2040:rp2040:rpipico,0,0.0,0,0.0,0,0.0,0,0.0,0,0.0,0,0.0,0,0.0,0,0.0,0,0.0,0,0.0,0,0.0,0,0.0,0,0.0,0,0.0