[Repo Assist] test: add NIST FIPS 180-4 SHA-256 test vectors

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Summary

Adds three additional SHA-256 test vectors from NIST FIPS 180-4 to tests/test_utils.c, strengthening confidence in the correctness of the freestanding SHA-256 implementation across all input sizes.

Changes

````tests/test_utils.c``` — extended test_sha256() with:

1. 56-byte padding-boundary vector — "abcdbcdecdefdefgefghfghighijhijkijkljklmklmnlmnomnopnopq" (56 bytes), which crosses the SHA-256 padding boundary (pad block needed). Expected: 248d6a61...db06c1

2. 112-byte two-block vector — "abcdefghbcdefghicdefghij..." (112 bytes), two full 512-bit processing blocks. Expected: cf5b16a7...fee9d1

3. 1,000,000 × 'a' via incremental API — 1000 calls of 1000 bytes each to l_sha256_update, exercising l_sha256_init / l_sha256_update / l_sha256_final with a very large input. Expected: cdc76e5c...2cd0

All 32 bytes of each expected hash are verified via l_memcmp.

Motivation

The existing tests only covered the empty string and "abc" (both fit in a single 512-bit block). These new vectors exercise:

• Inputs that require a second padding block (56–63 bytes)
• Multi-block inputs
• Large inputs accumulated through the incremental API

Test Status

✅ All tests pass on Linux x86_64 (gcc):

Testing l_sha256...
  [OK] sha256 nist 56-byte vector
  [OK] sha256 nist 112-byte vector
  [OK] sha256 nist 1M-a incremental
  L_Sha256 tests: PASSED

Full ./Taskfile test suite passes.

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