Add PforEncoding (Patched FOR) — claiming EncodingType::Pfor = 15

[han-yan01]

Summary:
PFOR is a sparse-outlier patched Frame-of-Reference encoder ported from MRS's AusList SOTA encoder portfolio (D97646777). Bitpacks the bottom 90th-percentile of residuals at base bitwidth, stores the top 10% as (position, full value) exception patches. Wins on columns with rare large outliers where a pure FOR would inflate bitwidth to cover the worst case.

Wire format: [encType:1B][dataType:1B][rowCount:4B][min:GV32][baseBitWidth:1B][numExceptions:varint][exceptionPositions:varint[]][exceptionValues:varint[]][bitpacked baseResiduals][7B zero-pad tail]

Decode follows the post-D98819389-style optimization: pass 1 bitunpacks all residuals via byte-aligned loadU64; pass 2 patches the exceptions in.

Slot allocation: claiming EncodingType::Pfor = 15. Per WS8 audit (T271476729), slots 12/13/14 are contested by 4+ in-flight diffs (D103045783 ChunkedBitPacking=12, D103975634 FOR=13/SubIntSplit=14, D105064770/D105134978 Fsst=12/14, plus implied ChunkedALP=13). 15 is the next free slot at time of commit. If a contending diff lands first claiming 15, this diff will need rebase to next free.

Initial readFactor: 1.5 (conservative). Prevents auto-selection by ManualEncodingSelectionPolicy until baked in. Mirrors ChunkedBitPacking pattern.

Stale-binary risk: xldb/disco/user_experiments/util/WriteConfigurationUtil.cpp persists EncodingType through Configerator. Older readers will throw NIMBLE_UNREACHABLE on EncodingType::Pfor=15 until they pick up this build — fail-loud, not silent.

Differential Revision: D105270894

[meta-codesync]

@han-yan01 has exported this pull request. If you are a Meta employee, you can view the originating Diff in D105270894.