fix(pipeline): absolutize project_dir to fix relative-path include doubling

[zackees]

Summary

fbuild test-emu .build/pio/<env> was failing every ESP32 QEMU CI run with fatal error: FastLED.h: No such file or directory even though the preceding ci-compile.py step had just produced a working firmware in the same project directory.

Issue described in #303.

Root cause

The CLI passes the project-dir argument through verbatim to the daemon, which builds a PathBuf from the string and hands it to the orchestrator unchanged. Whether the path is absolute or relative depends entirely on what the user typed.

• ci-compile.py invokes fbuild with an absolute project dir (workspace-rooted). discover_project_includes produces absolute include flags. zccache::path_arg_for_compile_cwd strips the compile-cwd prefix, leaving project-relative include flags that GCC resolves correctly against cwd = <project>/.
• fbuild test-emu .build/pio/<env> (the CI form) sends a relative path. discover_project_includes joins it as-is, producing relative include paths like -I.build/pio/esp32dev/lib/FastLED. zccache::path_arg_for_compile_cwd short-circuits on non-absolute inputs (if !path.is_absolute() { return path.to_string_lossy().to_string(); }) and passes them through unchanged. GCC then resolves the already-relative include path against the project-rooted compile cwd, producing the doubled prefix <project>/.build/pio/esp32dev/lib/FastLED -- which doesn't exist -- and FastLED.h is reported missing.

The bug had nothing to do with lib-discovery logic (the original hypothesis in #303). discover_project_includes already implements PlatformIO's lib/<Name>/src/ auto-discovery correctly. The output of that discovery was just spelled in a form that survived too far through the compile-cwd plumbing.

Fix

discover_project_includes now calls absolute_from_cwd(project_dir) at the top, making every emitted include dir absolute regardless of how the caller spelled the input. The normalize-then-exec chain then behaves identically for absolute and relative call sites.

absolute_from_cwd already existed in compiler.rs as pub fn absolute_from_cwd(path: &Path) -> PathBuf (used for source and output paths in compile_source to fix the analogous #282 bug). Promoted from private to pub for cross-module use.

Why this is the right layer

Three places could host the fix; pipeline-level is the smallest blast radius:

1. Daemon handlers (build, test_emu, deploy): would have to canonicalize on every handler; easy to miss new entry points.
2. zccache::path_arg_for_compile_cwd: would have to look up current_dir() for every relative path; couples the helper to process state.
3. discover_project_includes (chosen): one call, single responsibility (emit absolute include paths), already documents that callers should not depend on relativeness.

Test plan

• cargo test --lib -p fbuild-build pipeline::project_discovery -- 2/2 new regression tests passing
• cargo test --lib -p fbuild-build -- 546/546 passing
• CI: qemu_esp32dev_test, qemu_esp32s3_test, qemu_esp32c3_test should turn green once a release of fbuild ships and FastLED bumps its pin.

Fixes #303

Generated with Claude Code (https://claude.com/claude-code)

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ℹ️ Review info

⚙️ Run configuration

Configuration used: Path: .coderabbit.yaml

Review profile: CHILL

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Run ID: 60abb319-04a1-4e74-9988-c173ebda8ba5

📥 Commits

Reviewing files that changed from the base of the PR and between 8bfba3b and 24a671a.

📒 Files selected for processing (2)

• crates/fbuild-build/src/compiler.rs
• crates/fbuild-build/src/pipeline/project_discovery.rs

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch fix/lib-fastled-auto-discovery-303

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