Add fpc compression and decompression mutators

[batmac]

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[batmac]

Closing after evaluation — the code works (I merged main in, resolved the 16-month go.mod conflict, and verified byte-exact round-trips including through the stdin pipeline), but the value case does not hold up:

The algorithm is superseded. FPC is from 2007. The lineage since: Gorilla (2015) → Chimp/Chimp128 (2022) → Elf/Elf+ (VLDB 2023) → ALP (SIGMOD 2024), which is ~49% better than Gorilla and ~43% better than Chimp. None of those have a usable Go implementation: ALP is C++ (cgo would break our CGO_ENABLED=0 release builds), and the Gorilla Go ports are either stale since 2018 or archived.

A dependency-free alternative beats it. The classic byte-shuffle filter (transpose the 8 byte-planes of each float64 — what Blosc/HDF5/Parquet do) feeding our existing compressors, on 160 KB samples:

input	fpc	shuffle+zstd	shuffle+xz
random float64	143 KB	134 KB	128 KB
smooth series	112 KB	127 KB	106 KB

Shuffle+xz wins on both, needs no new dependency, is ~20 lines of pure Go, composes with every compressor ccat already has, and generalises to float32/int64/any fixed-width record via a shuffle:N argument — much more in the spirit of the mutator pipeline than a single-purpose codec.

So: not merging github.com/spenczar/fpc (57★, dormant since Oct 2023) for a narrower win. A shuffle/unshuffle mutator pair is the better direction.