[Repo Assist] fix: l_strtoul/l_strtoull return ULONG_MAX on negative overflow

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🤖 This is an automated pull request from Repo Assist.

Problem

l_strtoul and l_strtoull return the wrong value when both the overflow and negative sign conditions fire simultaneously — e.g.:

l_strtoul("-99999999999999999999999", NULL, 10)  // was: 1  should be: ULONG_MAX

C99 §7.20.1.4 requires these functions to return ULONG_MAX (or ULLONG_MAX) whenever the correct mathematical value cannot be represented, regardless of the sign of the input.

Root cause

When overflow is detected, the accumulator is clamped to ULONG_MAX. Then the return path applied a signed negation:

return neg ? (unsigned long)(-(long)acc) : acc;
```

`(long)ULONG_MAX` is implementation-defined (signed overflow). On typical two's-complement 64-bit hosts, GCC produces `(long)ULONG_MAX = -1`, so `-(long)(-1) = 1`, which is returned instead of `ULONG_MAX`.

The non-overflow negative path (e.g. `"-1" → ULONG_MAX`) was correct because `-(long)1 = -1` and `(unsigned long)(-1) = ULONG_MAX`. Only the overflow + neg combination was broken.

## Fix

Return `ULONG_MAX`/`ULLONG_MAX` immediately when the overflow flag is set, before the sign negation. Also replace the signed intermediate `-(long)acc` with unsigned subtraction `(0UL - acc)` to avoid implementation-defined behaviour on the non-overflow path.

## Test Status

All 9 test binaries pass on Linux x86-64. New regression tests added:

```
[OK] strtoul neg overflow -> ULONG_MAX   (strtoul("-99999999999999999999999") == ULONG_MAX)
[OK] strtoull neg overflow -> ULLONG_MAX (strtoull("-99999999999999999999999") == ULLONG_MAX)

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