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]>

docs/compatibility.md

@@ -11,6 +11,51 @@ mutually exclusive; the default is `vlibc`.
 | 4     | `--enable-spoof`     | `spoof`     | A glibc replica: emulates glibc for drop-in compatibility, higher than `muslext` or `vlibc` full. |
 | 5     | *(default)*          | `vlibc`     | glibc-extended: glibc minus its baggage, plus extended standard features. High (but not spoof-level) glibc compatibility. |
 
+## Profile mechanism
+
+The profiles map onto a single integer level, `VLIBC_LEVEL`. Levels are
+**cumulative**: a profile at level N exposes everything whose minimum level is
+`<= N`.
+
+| Level | Classification                          |
+|-------|-----------------------------------------|
+| 1     | ISO C + POSIX base                      |
+| 2     | + BSD / XSI extensions                  |
+| 3     | + GNU extensions                        |
+
+Public headers gate declarations with `#if VLIBC_LEVEL >= N`; a function whose
+minimum level is 2 is only declared when the active profile is level 2 or
+higher. The build applies the same rule to sources via
+`AM_CONDITIONAL([PROFILE_GE_2])` and `AM_CONDITIONAL([PROFILE_GE_3])`, so each
+level's implementation files are only compiled when the selected profile
+reaches that level.
+
+The generated installed header `include/vlibc/features.h` exposes the macros
+the gating is based on:
+
+- `VLIBC_LEVEL` — the active profile's integer level.
+- `VLIBC_PROFILE` — the selected profile name.
+- `VLIBC_LEVEL_GE(n)` — expands to 1 when `VLIBC_LEVEL >= n`, 0 otherwise.
+
+### Slice manifest
+
+The initial implementation covers eight string/memory functions, classified as
+follows:
+
+| Function     | Level | Origin  |
+|--------------|-------|---------|
+| `memcpy`     | L1    | ISO C   |
+| `memmove`    | L1    | ISO C   |
+| `memset`     | L1    | ISO C   |
+| `strlen`     | L1    | ISO C   |
+| `strcmp`     | L1    | ISO C   |
+| `strlcpy`    | L2    | BSD     |
+| `strlcat`    | L2    | BSD     |
+| `strcasestr` | L3    | GNU     |
+
+Profiles 4 and 5 (`spoof`, `vlibc`) build on the same level mechanism but are
+not implemented in this increment beyond their configure declarations.
+
 ## Differences from glibc
 
 - **No legacy baggage.** vlibc targets modern, standard-conforming behavior and