refactor: migrate MICRO_PIP to fine-grained feature flags

Replace monolithic MICRO_PIP/PIP_MICRO with granular flags:

Feature flags (CMake options + platform auto-detection):
- PIP_NO_FILESYSTEM, PIP_NO_THREADS, PIP_NO_SOCKET
- PIP_NO_PROCESS, PIP_NO_DYNLIB, PIP_NO_FFT, PIP_NO_SERIAL

Embedded optimization flag:
- PIP_EMBEDDED (auto-set for Pico SDK and FreeRTOS)
  Controls buffer sizes, time stubs, terminal fallback, init stubs

Platform blocks in CMakeLists.txt:
- Pico SDK: auto-disables FS, PROCESS, DYNLIB, FFT, SERIAL;
  conditionally disables THREADS (no FreeRTOS) and SOCKET (no LWIP)
- FreeRTOS: auto-disables FS, PROCESS, DYNLIB, FFT, SERIAL;
  conditionally disables SOCKET (no LWIP)
- Android: auto-disables PROCESS, DYNLIB, FFT

Updated 96 files across libs/, utils/, tests/, and CMakeLists.txt.
Builds verified for Linux (547 tests pass) and Pico SDK (100%).
Removed all MICRO_PIP and PIP_MICRO references (0 remaining).

libs/main/core/piinit.cpp

@@ -20,7 +20,7 @@
 #include "piinit.h"
 
 #include "piincludes_p.h"
-#ifndef MICRO_PIP
+#ifndef _PIP_INIT_STUB_
 
 #  include "pidir.h"
 #  include "piobject.h"
@@ -251,7 +251,7 @@ PIInit::PIInit() {
 		PIStringAscii("FreeBSD");
 #  elif defined(FREERTOS)
 		PIStringAscii("FreeRTOS");
-#  elif defined(MICRO_PIP)
+#  elif defined(_PIP_INIT_STUB_)
 		PIStringAscii("MicroPIP");
 #  else
 		uns.sysname;
@@ -395,4 +395,4 @@ __PIInit_Initializer__::~__PIInit_Initializer__() {
 	}
 }
 
-#endif // MICRO_PIP
+#endif // _PIP_INIT_STUB_