improve FSST implementation

[dfa1]

So far we had a very simple FSST implementation. Comparing our implementation against vortex-rust surfaced a lot of missed opportunities for vortex-java.

The idea is to reduce the gap by:

• studying the original paper: https://www.vldb.org/pvldb/vol13/p2649-boncz.pdf
• try to find a Java "zero deps" implementation
• if no candidates are found, try to isolate FSST into a separate module like in rust: https://docs.rs/vortex-fsst/latest/vortex_fsst/

Why? I will be faster to iterate on the design if the drop "vortex" shell and focus on the code algorithm.

[dfa1]

Done — closing this out. Summary of the rewrite:

Research: studied the paper in full (Boncz/Neumann/Leis, VLDB 2020) plus the actual spiraldb/fsst Rust crate (the reference implementation vortex-jni's own FSST is built on). Confirmed no zero-dependency Java FSST library exists, so per the issue's own decision tree, extracted FSST into its own module.

New standalone module: vortex-fsst — zero dependency on core/reader/writer, MemorySegment-native (matching this project's FFM-everywhere convention), independently testable/benchmarkable. Public API: Symbol, Compressor/CompressorBuilder, Decompressor.

Ported faithfully from the paper + the real reference's refinements:

• Branch-free O(1) matching (was up to 8 sequential hash probes per byte position)
• Adaptive byte-bounded sampling, per-generation min-count pruning, single-byte gain boost, final cost-based prune during training
• Unconditional-8-byte-store decode (Algorithm 1)

Measured result (JavaVsJniFsstBenchmark, unmodified before/after):

	Before	After	Speedup	Gap to vortex-jni
encode	0.085 ops/s	1.848 ops/s	~21.7x	36x slower → 1.6x slower
decode	5.243 ops/s	27.137 ops/s	~5.2x	6.5x slower → 1.3x slower

Along the way: a real data-corruption bug was found and fixed during review (the branch-free matcher could spuriously overrun the true end of short strings) — caught before it ever reached a merged PR, documented as a risk class in ADR 0022.

Merged as 8 PRs: #290, #291, #292, #293, #294, #295, #296, #297.

Explicitly out of scope (tracked in TODO.md): AVX512/SIMD kernel, true per-row lazy/random-access decode, OptFSST.