docs(profile): document gating mechanism, classification rule, slice manifest

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-openagent)

Co-authored-by: Sisyphus <[email protected]>

README.md

@@ -17,6 +17,13 @@ required capability.
      scripts and programs that rely on long-standing glibc behavior.
   5. *(default)* — **vlibc**: glibc-extended, without the spoofing layer, but
      with extended standard features and high glibc compatibility.
+- **Profiles 1-3 are functional.** `--enable-onlyposix`, `--enable-muslmimic`,
+  and `--enable-muslext` build with real build-time profile gating: a generated
+  `include/vlibc/features.h` exposes `VLIBC_LEVEL`, the public `<stddef.h>` and
+  `<string.h>` headers are gated by it, and an initial string slice (`memcpy`,
+  `memmove`, `memset`, `strlen`, `strcmp`, `strlcpy`, `strlcat`,
+  `strcasestr`) is compiled per level. Profiles 4 and 5 (`spoof`, `vlibc`)
+  remain configure-level declarations in this increment.
 - **Two install methods** (`--with-install=`):
   - `alongside` (default) — install next to the system libc, under a
     vlibc-specific tree; the system libc is left untouched.