[Repo Assist] feat: add l_crc32 and l_crc32_update — CRC-32/ISO-HDLC checksum

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Summary

Implements l_crc32 and l_crc32_update — CRC-32/ISO-HDLC (IEEE 802.3 / gzip / zip) checksums — as static inline functions in l_os.h.

Motivation

CRC-32 is the most widely used non-cryptographic checksum (gzip, zip, Ethernet frames, PNG). The library already has SHA-256, HMAC-SHA-256, and Base64. Adding CRC-32 completes the most common checksum/encoding needs without external dependencies.

API

/// One-shot CRC-32/ISO-HDLC checksum over len bytes of data.
unsigned int l_crc32(const void *data, size_t len);

/// Incremental update: pass crc=0 for the first chunk.
/// Feed subsequent chunks with the returned value.
unsigned int l_crc32_update(unsigned int crc, const void *data, size_t len);

Implementation

Bit-by-bit (8 iterations/byte), fully unrolled, reflected polynomial 0xEDB88320 (IEEE 802.3). No tables, no allocations, no initialization — pure static inline. On x86-64/AArch64, GCC/clang typically emit branchless CMOV/CSEL for the inner loop.

Tests

Six new tests in tests/test_utils.c:

Test	Value
RFC 3720 check value "123456789"	0xCBF43926
Empty string	0x00000000
"abc"	0x352441C2
"The quick brown fox..."	0x414FA339
Two-chunk incremental matches single-shot	✅
Single zero byte 0x00	0xD202EF8D

Test Status

✅ ./Taskfile test — all non-infrastructure tests pass. (BearSSL submodule absent in this environment — pre-existing, not caused by this change.)

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