Build SAMD51J fails: get_core_dir(adafruit) points at nonexistent cores subdir

[zackees]

Symptom

Build SAMD51J workflow fails on every main build with:

build error: build failed: compilation failed for
  .fbuild/prod/cache/platforms/adafruit-ArduinoCore-samd/.../1.7.16/ArduinoCore-samd-1.7.16/variants/feather_m4/variant.cpp:
In file included from variants/feather_m4/variant.cpp:19:
variants/feather_m4/variant.h:43:10: fatal error: WVariant.h: No such file or directory
   43 | #include "WVariant.h"

Reproducible: gh run view 26694160364 --log-failed and every recent SAMD51J workflow run on main.

Root cause

The board JSON for adafruit_feather_m4 (and the rest of the Adafruit SAMD family) declares build.core = "adafruit". This matches PlatformIO's upstream board JSON for the same board — so the board-validator agrees.

fbuild's ArduinoCore::get_core_dir(core_name) literally joins cores/<core_name>:

pub fn get_core_dir(&self, core_name: &str) -> PathBuf {
    self.get_cores_dir().join(core_name)
}

So it asks for <framework>/cores/adafruit/. But the adafruit-ArduinoCore-samd repo at v1.7.16 only has cores/arduino/ (verified via GitHub API: GET /repos/adafruit/ArduinoCore-samd/contents/cores?ref=1.7.16 returns one entry, name="arduino"). There is no cores/adafruit/.

PlatformIO works around this in its atmelsam builder by interpreting build.core differently — it doesn't blindly use it as the cores/ subdir name. fbuild just joins.

Result: variant.h is found (it's in variants/feather_m4/), but its #include "WVariant.h" (which lives in cores/arduino/WVariant.h) is unreachable because the include-path entry points at the nonexistent cores/adafruit/ instead.

Proposed fix

In ArduinoCore::get_core_dir(core_name) (or equivalent in the SAM orchestrator's include-path assembly), fall back to cores/arduino/ when cores/<core_name>/ doesn't exist on disk:

pub fn get_core_dir(&self, core_name: &str) -> PathBuf {
    let primary = self.get_cores_dir().join(core_name);
    if primary.is_dir() {
        return primary;
    }
    // Many Arduino-compatible cores declare build.core = "<vendor>" in
    // their board JSON for branding purposes but ship the actual source
    // in cores/arduino/. Mirror PlatformIO's atmelsam builder fallback
    // when the named directory doesn't exist.
    let fallback = self.get_cores_dir().join("arduino");
    if fallback.is_dir() {
        return fallback;
    }
    primary
}

Or, more surgically, do the fallback only in sam/orchestrator.rs since SAM is the only confirmed affected platform.

Affected boards

Likely all Adafruit SAMD boards (feather_m4, itsybitsy_m4, metro_m4, trinket_m0, ...) and any other vendor that uses this branding pattern in build.core.

Test plan

• Unit test for get_core_dir fallback: given a synthetic framework dir with cores/arduino/ but no cores/adafruit/, get_core_dir("adafruit") should return .../cores/arduino/.
• Integration: Build SAMD51J workflow turns green on next main push.
• Spot-check: Build SAMD21, Build SAMD21 Zero, Build SAMD51P workflows (haven't run recently — likely also red).

Filed during a CI-green sweep on fbuild main.