fast-crc32 provenance

I vendor the generator and license files from corsix/fast-crc32 at 13f5289ceb6065d014f9e1e64f161ea7a926c3b7. These four files are unchanged from that revision:

File	SHA256
`generate.c`	`d6980f19f0d814ffb9cf2a48692c13b2ae36d04325ff93f411498e5aa2329bf1`
`LICENSE.md`	`16627d1e08d5f2204b78394098eeec9c49a8a1de63c6dc144d87e6117facefb3`
`LICENSE.MIT.md`	`2a089b18ed6324886d134edbf9c5e9cde62e13458f30a10200a1f78089f90be7`
`LICENSE.zlib.md`	`715a5af28f2eb61eae78b857a77d796787ccc71b31ead1078aa5fc9e9727f900`

The generator explicitly applies its MIT-or-zlib choice to both the program and its output. I preserve those terms in the generated files; they are not relicensed as Everett-authored code. I do not vendor the upstream benchmark harness or its separate third_party collection.

Regeneration

From an Everett source checkout, with Python 3 and a host C compiler available:

python3 tools/generate_crc32c.py

python3 tools/generate_crc32c.py --check

The script compiles the pinned generator in a temporary directory and selects CRC32C's Castagnoli polynomial for each backend. Its adaptations give the C output C++ header linkage and separate namespaces, use alias-safe scalar loads, adjust C literals, and scope helper macros. Each generated file records its exact ISA and algorithm arguments. Regeneration is a developer operation; it is not part of configuration, compilation, installation or ordinary consumption.

I check in all generated backends under include/everett/detail/ and install them with the headers. The generator and script remain in the source checkout; the installed package needs neither. This provenance document and all three upstream license files are installed together so their relative links remain valid.

The available backends include a portable fallback, ARM variants and x86 variants. Selection depends on the compiler and available target features; shipping a generated backend does not establish that it has been executed or benchmarked on every platform.