Make declared-supported PLATFORMIO_* overrides actually affect native config resolution

[zackees]

Parent: #34

Problem

PlatformIOConfig::from_path_with_overrides() stores a PioEnvOverrides snapshot and its docs say getters should honor caller-provided PLATFORMIO_* overrides. In practice, only some getters do. Several build-relevant getters appear to ignore the attached overrides entirely.

Relevant repo evidence

• PlatformIOConfig stores overrides and documents that getters should consult them: crates/fbuild-config/src/ini_parser.rs:27-47
• get_build_flags() reads only config.get("build_flags"): crates/fbuild-config/src/ini_parser.rs:123-135
• get_build_src_flags() reads only config.get("build_src_flags"): crates/fbuild-config/src/ini_parser.rs:137-143
• get_default_environment() reads only [platformio].default_envs: crates/fbuild-config/src/ini_parser.rs:95-121
• PioEnvOverrides exposes accessors for PLATFORMIO_BUILD_FLAGS, PLATFORMIO_BUILD_SRC_FLAGS, PLATFORMIO_BUILD_UNFLAGS, PLATFORMIO_BUILD_SRC_FILTER, and PLATFORMIO_DEFAULT_ENVS: crates/fbuild-config/src/pio_env.rs:111-125

Why this matters

The CLI forwards PLATFORMIO_* env vars to the daemon specifically so the daemon can behave like PlatformIO without inheriting the caller environment. If the getters do not use those forwarded values, native builds can diverge from PlatformIO even before we get into SCons/advanced scripting territory.

Requested change

Audit all declared supported PLATFORMIO_* overrides and make the actual config getters honor them consistently, or downgrade any unimplemented ones to warning-only/unsupported status.

Priority candidates

• PLATFORMIO_BUILD_FLAGS
• PLATFORMIO_BUILD_SRC_FLAGS
• PLATFORMIO_BUILD_UNFLAGS
• PLATFORMIO_BUILD_SRC_FILTER
• PLATFORMIO_DEFAULT_ENVS

Acceptance criteria

• Every env var listed as supported either changes runtime behavior or is reclassified
• Tests cover forwarded overrides through PlatformIOConfig::from_path_with_overrides()
• The supported/no-op/unsupported classification in pio_env.rs matches reality