feat: POSIX libc surface, native binutils (as/ld/ar run on MontaukOS)

Grow the Montauk libc enough to cross-build binutils 2.43.1 with
--host=x86_64-montauk. gas, ld, ar, nm, objcopy, objdump, readelf,
ranlib, strip and friends now link as native ET_EXEC Montauk binaries
(staged stripped in toolchain/native/, not yet shipped in the image).

New libc surface: full Linux errno and signal sets, O_* flags with
real O_EXCL/O_APPEND semantics, unlink/rmdir/dup/dup2/getpid/_exit,
kill (SYS_KILL), fcntl, fileno/fdopen, putc/getchar/rewind,
ctime/asctime, strtoll/strtoull/atoll, bsearch, mkstemp/mktemp,
realpath (lexical, drive-prefix aware), mbstowcs/mblen, chmod/fchmod/
umask/utime no-ops (VFS has no modes or settable times), full struct
stat with fake inodes, sscanf field widths (bounded conversions),
SCN*/PRI* completions, wait/waitpid over SYS_WAITPID, and new headers
sys/wait.h, sys/param.h, utime.h, wchar.h, memory.h.

fork/exec/pipe are declared but fail with ENOSYS: binutils never
spawns, and real process plumbing is the posix_spawn milestone, which
needs kernel support (exit status reporting, fd redirection).

TCC's montauk_compat.h shims (unlink, chmod, execvp, realpath, fdopen,
strtoll, strtoull) are retired in favor of the libc versions.

Co-Authored-By: Claude Fable 5 <[email protected]>

toolchain/README.md

@@ -61,3 +61,36 @@ toolchain/
   local/                       # install prefix (ignored)
   sysroot/                     # generated target sysroot (ignored)
 ```
+
+## Native binutils (runs on MontaukOS)
+
+Binutils can be cross-compiled to run *on* MontaukOS itself
+(`--host=x86_64-montauk`), the first step toward a self-hosted GCC:
+
+```bash
+mkdir -p toolchain/build/binutils-native && cd toolchain/build/binutils-native
+export PATH=$PWD/../../local/bin:$PATH
+../../src/binutils-2.43.1/configure \
+    --build=x86_64-pc-linux-gnu --host=x86_64-montauk \
+    --target=x86_64-montauk --prefix=/usr \
+    --disable-nls --disable-werror --disable-gprofng --disable-gold \
+    --disable-plugins --disable-shared --enable-static
+make -j$(nproc) all-gas all-ld all-binutils
+make install-strip-gas install-strip-ld install-strip-binutils \
+    DESTDIR=$PWD/../../native
+```
+
+Notes:
+- Build only `all-gas all-ld all-binutils`. gprof needs fscanf with
+  %[] scansets (not in the Montauk libc); gold and gprofng are
+  disabled outright.
+- The libc gained a large POSIX surface for this port (fd functions,
+  full errno/signal sets, struct stat, sys/wait.h, utime.h, wchar.h,
+  sys/param.h, memory.h, bsearch, sscanf field widths, ...). pex-style
+  process spawning (fork/exec/pipe) is stubbed to fail with ENOSYS:
+  the tools themselves never spawn, and real process plumbing is the
+  posix_spawn milestone that needs kernel support (exit codes in
+  SYS_WAITPID, fd redirection wiring).
+- The staged tree in toolchain/native/ is not yet integrated into the
+  OS image; that needs a target-side sysroot layout decision (where
+  headers, libc.a and crt objects live on 0:/).