sprint_event: avoid %hu format (unsupported by MicroPython internal printf)

[bwhitman]

Problem

sprint_event in src/patches.c printed the osc field with PRIu16, which expands to %hu. MicroPython's internal printf (mp_vprintf) has no h length-modifier support, so in any MicroPython port where MICROPY_USE_INTERNAL_PRINTF=1 overrides snprintf (the default; micropython/shared/libc/printf.c routes it into mp_vprintf), formatting reaches assert(*fmt == '%' || !"unsupported fmt char") in py/mpprint.c and aborts on debug builds — triggered by anything that serializes events: tulip.amy_dump_state(), amyboard.update_sketch_knobs(), etc.

Affected in practice: Tulip Linux desktop builds and the AMYboard VCV Rack plugin build. Tulip macOS desktop happens to be immune: Darwin's fortify headers rewrite snprintf calls to ___snprintf_chk (system libc) regardless of -U _FORTIFY_SOURCE, bypassing the MicroPython override — verified live at the desktop REPL.

Fix

Print the osc field with %u and an (unsigned) cast (varargs promote uint16_t to int anyway, so this is value-identical). These were the only two %hu sites reachable through snprintf in AMY; all other formats used (%d/%u via 32-bit PRI macros, %.3f, %s) are supported by mp_vprintf.

Testing

make test: 109 tests pass.

🤖 Generated with Claude Code

[github-actions]

🎛️ AMY HW CI (AMYboard bench)

Flashed this PR's AMY (LoadTestChord: 6-voice Juno patch=1, one held note every 2 s) onto the physical AMYboard and measured the smoothed render load as the chord grows — back-to-back with the same sketch built at the PR's merge base, so Δ is this PR's own cost.

✅ PASS — the bench ran the test to completion.

notes held	main @ 4885773	this PR	Δ
1	1602	1601	-1
2	1799	1800	+1
3	2480	2488	+8
4	2720	2726	+6
5	3465	3470	+5
6	3673	3679	+6

Full chord settled render μs: 3676 (was 3672, Δ +0.1%) (peak 3691, 39 samples)

⬇️ Artifacts: serial log · load trace · report

_{Self-hosted bench (amyboardci). FAIL means only that the test could not run — the load values are informational, with no threshold and no audio compare. See tools/arduino_loadsweep/.}