feat(tsdb): add integer codec stages for ES94 numeric pipeline

[salvatore-campagna]

Summary

This PR adds the concrete stage implementations for the ES94 Deepstore Pipeline Codec. PR 1 (#143589) established the pipeline framework (type system, wire format, metadata I/O, block format, context objects). This PR builds on that base with the four core numeric codec stages: delta, offset, GCD, and bit-pack.

What's included

Stage contracts

Component	Purpose
NumericEncoder	In-place transform contract for encoding - no DataOutput, metadata via context.metadata()
NumericDecoder	In-place reverse transform contract for decoding - reads metadata, throws IOException
NumericCodecStage	Combined encoder/decoder interface for stateless singleton transform stages
PayloadEncoder	Terminal stage contract for serializing values to bytes via DataOutput
PayloadDecoder	Terminal stage contract for deserializing bytes back to values via DataInput
PayloadCodecStage	Combined encoder/decoder interface for terminal payload stages

Stage implementations

Stage	Type	ID	Description
DeltaCodecStage	Transform	0x01	Backward difference for monotonic sequences. Stores first value as ZLong metadata
OffsetCodecStage	Transform	0x02	Subtracts minimum value when significant relative to range. Stores min as ZLong metadata
GcdCodecStage	Transform	0x03	Divides by GCD when > 1 (unsigned). Stores gcd - 2 as VLong metadata
BitPackCodecStage	Payload	0xA1	Terminal bit-packing via DocValuesForUtil. Writes VInt(bitsPerValue) + packed data directly to stream

Design highlights

• Transform vs payload separation: transform stages (NumericCodecStage) modify long[] in-place and write metadata to an in-memory buffer; the payload stage (PayloadCodecStage) writes directly to the byte stream, matching the block layout from PR 1: [bitmap][payload][stage metadata]
• Stateless singletons: all three transform stages use the INSTANCE pattern with private constructors - no mutable state, safe to share across threads. BitPackCodecStage is a record taking DocValuesForUtil as its only parameter
• Skip heuristics: each transform stage decides whether to apply itself (delta skips non-monotonic sequences, offset skips when min is zero or small relative to max, GCD skips when the divisor is <= 1). When a stage skips, values are unchanged and the position bitmap is not set
• SIMD-friendly hot loops: DeltaCodecStage.isMonotonic uses branchless conditional adds instead of if/else chains; OffsetCodecStage.encode uses Math.min/Math.max intrinsics for min/max computation - both patterns enable JIT auto-vectorization
• Power-of-two GCD optimization: when the GCD is a power of two, division and multiplication are replaced by arithmetic shifts (1 cycle vs ~100 cycles for idiv), which are also SIMD-friendly
• All-zeros bitpack optimization: when all values are zero, bitsPerValue is written as 0 and ForUtil encoding/decoding is skipped entirely (aligned with the existing TSDBDocValuesEncoder)
• Wire format diagrams: each stage's Javadoc includes a <pre> diagram showing the byte-level metadata/payload layout and where it lives within the block structure

Testing

Two abstract base classes provide reusable test infrastructure:

Base class	Purpose
AbstractTransformStageTestCase	assertStageSkipped, assertTransformRoundTrip, assertMultiBlockTransformRoundTrip, monotonic generators
AbstractPayloadStageTestCase	assertPayloadRoundTrip (full and partial block), assertMultiBlockPayloadRoundTrip, randomValueWithExactBits

Multi-block tests decode multiple blocks sequentially into a reused array (pre-filled with Long.MAX_VALUE) to verify no stale data leaks between blocks.

./gradlew :server:test --tests "org.elasticsearch.index.codec.tsdb.pipeline.*"

[elasticsearchmachine]

Pinging @elastic/es-storage-engine (Team:StorageEngine)

[martijnvg]

Looks good @salvatore-campagna, 👍 .