fix(esp32): pass PlatformIO's SDK defines on 2.x-layout frameworks

[zackees]

Fixes #1549. Found while benchmarking #1537, whose pinned stack is exactly this layout.

Problem

arduino-esp32 2.x SDKs (tools/sdk/<mcu>/ with no flags/, e.g. espressif32@6.13.0) ship no flags/defines. get_sdk_defines therefore returned nothing, and every TU was compiled without the SDK CPPDEFINES PlatformIO passes: ESP_PLATFORM, IDF_VER, MBEDTLS_CONFIG_FILE, _GNU_SOURCE, HAVE_CONFIG_H, WITH_POSIX, UNITY_INCLUDE_CONFIG_H, _POSIX_READER_WRITER_LOCKS.

The visible symptom: FastLED does not compile. taskENTER_CRITICAL(&mux) only takes an argument #ifdef ESP_PLATFORM, so the build fails with macro "taskENTER_CRITICAL" passed 1 arguments, but takes just 0.

Fix

• When flags/defines is absent, get_sdk_defines falls back to the CPPDEFINES block of tools/platformio-build-<mcu>.py, the same source parse_pio_cpppath already uses for include dirs.
• It emits only literal, SDK-level entries, in the raw flags/defines form (-DIDF_VER=\"v4.4.7-dirty\").
• It skips entries computed from the SCons env ($BOARD_F_CPU, %-formatted), and the Arduino/board-level names (ARDUINO*, ESP32, F_CPU) that fbuild already derives from the board. Duplicating those would trigger "redefined" warnings.
• Fast path: Esp32FingerprintMetadata now includes the SDK defines. Before, the fingerprint ignored them (a user_flags vector was built and then dropped), so a build after this change, or after any framework change to defines, replayed the old firmware until a source file changed. That was reproduced locally before the fix.

Out of scope, noted: PlatformIO's script also sets ARDUINO=10812 (fbuild passes 10808) and ARDUINO_PARTITION_<name>. Those are board-level and unchanged here.

Verification

• FastLED DemoReel100 on esp32s3 / espressif32@6.13.0 now builds with no build_flags workaround.
• bench/blink: the first build after the change recompiles with all 8 SDK defines, and the next warm build is 51 ms (fast path).
• Tests:
  • pio_cppdefines_yield_the_sdk_defines_in_flags_defines_form runs against the real block;
  • fallback-without-flags/defines and flags-file-wins cases;
  • sdk_defines_change_fast_path_metadata.

[coderabbitai]

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📥 Commits

Reviewing files that changed from the base of the PR and between 83dc147 and 0b8cf81.

📒 Files selected for processing (5)

• crates/fbuild-build-esp/src/esp32/orchestrator/build.rs
• crates/fbuild-build-esp/src/esp32/orchestrator/fingerprint.rs
• crates/fbuild-library/src/library/esp32_framework/parsing.rs
• crates/fbuild-library/src/library/esp32_framework/sdk_paths.rs
• crates/fbuild-library/src/library/esp32_framework/tests.rs

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