[Repo Assist] improve: add l_stpcpy, l_stpncpy, l_memccpy — POSIX string copy helpers

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Summary

Add three POSIX string utility functions missing from l_os.h:

Function	Signature	Returns
l_stpcpy	char *stpcpy(char *dst, const char *src)	Pointer to terminating \0 in dst
l_stpncpy	char *stpncpy(char *dst, const char *src, size_t n)	dst + n always
l_memccpy	void *memccpy(void *dst, const void *src, int c, size_t n)	Pointer past c in dst, or NULL

Motivation

l_stpcpy avoids a second l_strlen pass when building a string by successive copies — the returned end-pointer feeds directly into the next copy, giving O(n) concatenation:

char *p = buf;
p = l_stpcpy(p, first);
p = l_stpcpy(p, " ");
p = l_stpcpy(p, last);
```

`l_memccpy` is the bounded counterpart that stops at a sentinel byte, avoiding a separate `l_memchr` pass for delimiter-separated byte streams.

## Changes

- **`l_os.h`**: forward declarations + implementations after `l_strncat`; `#define` aliases in `#ifndef L_DONTOVERRIDE` block.
- **`tests/test_strings.c`**: `test_stpcpy_stpncpy_memccpy()` covering copy correctness, pointer chaining, zero-padding, truncation, found/not-found, and `n=0` edge cases.
- **`docs/`**: regenerated API.md, COMPAT.md, COVERAGE.md.

## Test Status

```
Linux (gcc)   Build  PASS
Linux (gcc)   Test   PASS
Linux (gcc)   Verify PASS
Linux (clang) Build  PASS
Linux (clang) Test   PASS
Linux (clang) Verify PASS
Docs          Generate PASS

ARM/AArch64 cross-compilers not available in this environment.

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