fix: use PlatformIO platform API to locate boot_app0.bin

[Copilot]

FRAMEWORK_DIR is not exposed as a SCons env variable in post-build scripts — it's internal to the platform builder. The previous fix always resolved to "", so boot_app0.bin was never copied and remained absent from CI artifact zips.

Changes

• pio-scripts/output_bins.py: Replace env.get("FRAMEWORK_DIR", "") with env.PioPlatform().get_package_dir("framework-arduinoespressif32") — the correct PlatformIO API, consistent with how obj-dump.py already resolves platform paths. ESP8266 builds return None from get_package_dir() and are silently skipped; old arduino-esp32 1.0.x (no tools/partitions/boot_app0.bin) is skipped by the isfile() guard.

_ARDUINO_ESP32_PACKAGE = "framework-arduinoespressif32"
try:
    framework_dir = env.PioPlatform().get_package_dir(_ARDUINO_ESP32_PACKAGE)
    if framework_dir:
        boot_app0_src = os.path.join(str(framework_dir), "tools", "partitions", "boot_app0.bin")
        if os.path.isfile(boot_app0_src):
            shutil.copy(boot_app0_src, os.path.join(builddir, "boot_app0.bin"))
except Exception as e:
    print(f"Warning: could not copy boot_app0.bin ({e})")

• .github/workflows/build.yml: Updated stale comment to reflect the platform API approach.

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