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]>

STYLEGUIDE.md

@@ -76,6 +76,34 @@ linked binaries (see `docs/overview.md`):
 
 Every public function carries the tightest correct attribute.
 
+## Profile gating
+
+Profile-gated code follows these formatting and naming conventions (the
+semantic classification of functions into levels lives in
+`docs/compatibility.md`, not here):
+
+- Public headers gate declarations with `#if VLIBC_LEVEL >= N`. Prefer the
+  `VLIBC_LEVEL_GE(n)` helper from `include/vlibc/features.h`; fall back to the
+  raw comparison only when the level is a literal constant:
+
+  ```c
+  #if VLIBC_LEVEL_GE(2)
+  size_t strlcpy(char *dst, const char *src, size_t n);
+  #endif
+  ```
+
+- Group implementation sources by level in `Makefile.am` using the per-level
+  conditionals `if PROFILE_GE_2` / `if PROFILE_GE_3`, mirroring the
+  `#if VLIBC_LEVEL >= N` rule in headers.
+- Standard headers use `VLIBC_`-prefixed include guards
+  (`VLIBC_STDDEF_H`, `VLIBC_STRING_H`), never the `_STDDEF_H`-style reserved
+  forms. A leading underscore is reserved for the implementation and the
+  C/POSIX standards (see Naming); the `VLIBC_` prefix stays out of the
+  reserved namespace while remaining unambiguous.
+- Intent attributes respect the gating: `pure` / `const` are only valid on
+  side-effect-free functions. Never annotate a mutating function (`memcpy`,
+  `memmove`, `memset`, `strlcpy`, `strlcat`) with either.
+
 ## Error handling
 
 - No empty blocks; no silent failure.