feat: add FSST12 (12-bit-code) variant

[mprammer]

Adds the FSST12 codec as a sibling module to the classic 8-bit codec, matching the variant defined in the cwida/fsst reference implementation: 12-bit codes (4096 entries), first 256 reserved as single-byte identity codes, no escape mechanism, output bit-packed at 1.5 bytes per code. Closes #142. Public surface is fsst::fsst12::{Compressor12, CompressorBuilder12, Decompressor12} mirroring the classic API; the classic 8-bit codec is unchanged.

🤖 Generated with Claude Code

[codspeed-hq]

Merging this PR will not alter performance

✅ 25 untouched benchmarks
🆕 17 new benchmarks

Performance Changes

	Mode	Benchmark	BASE	HEAD	Efficiency
🆕	Simulation	compress	N/A	2.2 ms	N/A
🆕	Simulation	decompress	N/A	1.1 ms	N/A
🆕	Simulation	compress	N/A	10.8 ms	N/A
🆕	Simulation	decompress	N/A	5.6 ms	N/A
🆕	Simulation	1mb-abcdefgh	N/A	10.1 ms	N/A
🆕	Simulation	train-and-compress	N/A	22.3 ms	N/A
🆕	Simulation	compress-only	N/A	31.3 ms	N/A
🆕	Simulation	decompress	N/A	15 ms	N/A
🆕	Simulation	train-and-compress	N/A	41.5 ms	N/A
🆕	Simulation	compress-only	N/A	17.5 ms	N/A
🆕	Simulation	decompress	N/A	8.2 ms	N/A
🆕	Simulation	train-and-compress	N/A	29.9 ms	N/A
🆕	Simulation	compress-only	N/A	3.3 µs	N/A
🆕	Simulation	decompress	N/A	2.3 µs	N/A
🆕	Simulation	train-and-compress	N/A	7.7 ms	N/A
🆕	Simulation	compress-only	N/A	11.9 ms	N/A
🆕	Simulation	decompress	N/A	5.6 ms	N/A

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_{Comparing mp/fsst12 (17bfe80) with develop (76baf7a)}