Add emulated SSD1306 render test

[laurb9]

Follow-up to #32: the OLED itself is now emulated, so tests verify actual rendered pixels, not just the serial trace.

How it works

A small C runner (test/sim_display/sim_display.c, linked against Ubuntu jammy's libsimavr-dev 1.6) loads .pio/build/uno_sim/firmware.elf, attaches simavr's ssd1306_virt part over TWI at 0x3C, and steps the CPU until the firmware prints sim: TESTS COMPLETE — which lands right after the 5th display.display(), making the framebuffer snapshot deterministic. VRAM is dumped as 64x128 ASCII art ('#'/'.') and compared pixel-for-pixel against the golden test/uno-display.txt (which is git-diffable and human-reviewable — it clearly shows the rendered voltmeter screen).

make sim-display-test          # build + render + compare
make sim-display-test-update   # regenerate the golden after intentional UI changes

Licensing note

ssd1306_virt.{c,h} are vendored verbatim from simavr v1.6 examples/parts and are GPLv3 — test tooling only, compiled solely into the host-side runner, never linked into the MIT firmware. Documented in test/sim_display/README.md.

Changes

• test/sim_display/ (runner + vendored part + README), test/sim-display-run.sh, golden baseline
• Makefile targets; devcontainer gains gcc libsimavr-dev libelf-dev; display test added to the existing CI sim-test job (no duplicate pio builds)
• AGENTS.md + verify skill updated

Verification (devcontainer)

• make sim-display-test passes; second run identical (deterministic)
• Negative test: single flipped pixel in the golden → FAIL with diff (comparison is real)
• make sim-test still passes; all 5 pio envs build; devcontainer image builds

Found in passing: the MODE_BUTTON_PIN redefined warning in PlatformIO build logs is an artifact of PIO's .ino conversion pass (it registers all #defines while passing conditionals through unevaluated) — the compiled code is correct on every board. Cosmetic fix in a separate PR.

Agent: claude-opus-4-8 implement / claude-fable-5 validate, claude-sonnet-5 verify (claude-code)

🤖 Generated with Claude Code

[laurb9]

Closing per maintainer decision: ssd1306_virt.{c,h} are GPLv3 and this repo is MIT — vendoring license-incompatible source isn't acceptable here, even as test-only tooling.

If display-level testing is ever wanted again, two GPL-free-repo approaches exist:

1. Fetch at test time: keep only MIT runner code in the repo and have the Makefile/CI download ssd1306_virt from simavr's GitHub at a pinned commit — GPL source never enters this repo (the non-distributed test binary still links GPL libsimavr, like the existing sim-test uses the GPL simavr executable).
2. Clean-room MIT part: the SSD1306 command subset the firmware uses (addressing mode, page/column ranges, display on/off, data writes) is small enough for a ~150-line MIT implementation.

The serial-trace smoke test (make sim-test) is unaffected and remains in place.

Agent: claude-fable-5 (claude-code)