docs: record the deferred native-toolchain workstream

The config comment referencing this file was dangling. Captures the
twelve gcc-native and four rust-native findings so the work is not lost:
the GCC 13 -> 16 move and why (char8_t, libcody's C++11-only probe, the
cpuid macros GCC 15/16 removed, then an ICE), the stale same_as symlink,
the gnu17 exemption, the missing relibc dependency, the getopt.h
shadowing, and the fnmatch extensions implemented in the fork.

Records two things explicitly because they are easy to get wrong again:

- The old 'Redox C++/pthread header gaps' justification was false.
  llvm-native cooks clean in isolation; it only failed in parallel builds
  because of a cookbook race on shared staging paths. A build-system bug
  had been misdiagnosed as a porting gap and used to exclude a required
  package.

- My own 'relibc fenv.h is a stub' diagnosis was also false. relibc
  delegates to openlibm_fenv.h which defines both types; the header just
  was not reaching the sysroot. Generalised into a rule: an undeclared
  identifier against relibc is more often a header-path problem than a
  missing implementation.

Also notes the exact stopping point (host libstdc++ headers leaking into
a target compile), the re-enable procedure, and the advice to diagnose
with COOKBOOK_COOK_JOBS=1 since several wrong turns came from attributing
one recipe's error to another in the interleaved parallel log.

docs/README.md

@@ -90,6 +90,7 @@ Do not flatten those into one “supported” claim in public summaries.
 
 - `../local/docs/SCRIPT-BEHAVIOR-MATRIX.md` — what the main sync/fetch/apply/build scripts do and do not guarantee
 - `../local/docs/FORK-BUMP-PATCHING-POLICY.md` — delicate patching during a version or toolchain bump (a bump is a rebase, never a replacement)
+- `../local/docs/NATIVE-TOOLCHAIN-WORKSTREAM.md` — gcc-native/rust-native: deferred workstream, findings and re-enable procedure
 
 ## Current State Summary (as of 2026-07-18)