fix(build): escaped quotes in build_flags defines are mangled before reaching gcc (ESP32 sketch compile) #947

Context

NightDriverStrip env:demo sets build_src_flags = ... -DPROJECT_NAME="\"Demo\"" (standard PlatformIO idiom for a string-valued define — SCons passes flags through a shell layer, so gcc receives the argv element -DPROJECT_NAME="Demo").

Under fbuild (Rust, ESP32 orchestrator, compile-sketch phase), every use of PROJECT_NAME fails with:

<command-line>: error: unable to find string literal operator 'operator""Demo' with 'const char [1]', 'unsigned int' arguments
src/effectmanager_persistence.cpp:262:31: note: in expansion of macro 'PROJECT_NAME'

i.e. the macro value reaching gcc is ""Demo""-shaped — backslashes stripped, literal quotes kept. fbuild-config's parse_flags preserves the token verbatim, so the corruption happens downstream in flag re-tokenization (fbuild_core::shell_split::split deliberately treats \ as literal and strips quote chars — correct for Windows paths, wrong for escaped-quote defines) or in the compile-command assembly.

Reproduced in the #942 Docker profiling harness: cold fbuild build /tmp/nds -e demo (NightDriverStrip @ 5911331) fails after ~11.5 min in compile-sketch. This blocks the #942 Phase 1 baseline (and any real project using string-valued defines, which is a very common PlatformIO pattern).

Acceptance criteria

• -DNAME="\"Value\"" in build_flags/build_src_flags reaches the compiler as the argv element -DNAME="Value" (direct-exec semantics), on the ESP32 path at minimum.
• Unit test covering escaped-quote defines through the flag pipeline (INI → argv).
• Audit of the other orchestrators' (esp8266, rp2040, stm32) use of shell_split for the same hazard — fix or file follow-ups.
• NightDriverStrip env:demo compiles past effectmanager_persistence.cpp in the meta: profiling-driven build performance burndown (Docker Linux, NightDriverStrip, cold vs hot cache) #942 harness.

Decisions

• Severity: HIGH — user-facing build failure on a very common flag idiom.
• Fix shape: make the flag tokenizer shell-unescape \" inside double-quoted regions (or bypass re-tokenization for already-tokenized flags) while keeping backslashes literal outside quotes so Windows paths stay safe. Exact location pending trace.

Part of #942 (blocks Phase 1 baseline).