feat: add FSST12 (12-bit-code) variant #216

Performance Gate Passed

✅ 25 untouched benchmarks
🆕 17 new benchmarks

	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

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