perf(storage): prototype compact PieceV2 storage

[Kubuxu]

This is largely LLM code. I haven't yet fully reviewed it myself, so some LLM-isms might be there.
Published because results are encouraging.
I will do a detailed review before anyone else.

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Summary

Draft implementation of the compact two-slot PieceV2 storage prototype, related to #286.

New compact-state datasets store pieces in a contiguous per-dataset array:

struct PieceV2 {
    bytes32 root;
    uint256 metadata;
}

metadata packs padding, tree height, leaf count, and the Fenwick partial sum into one storage slot.

Changes

• Hardened canonical PieceCIDv2 validation and decoded (padding, height, root) in one pass.
• Added compact PieceV2 storage while retaining the existing declarations for physical layout safety.
• Moved piece addition, proof verification, lookup, getters, pagination, CID search, scheduling, removal, and cleanup to compact storage.
• Preserved piece IDs, public ABI behavior, events, and listener callback arguments for compact-state datasets.
• Added coverage for packing bounds, Fenwick semantics, proofs, pagination, removal, cleanup, and raw storage reclamation.
• Measured steady-state addPieces storage activity for 1, 4, 16, and 32-piece batches.

Storage measurements

Each scenario creates a fresh dataset, adds one seed piece before recording, then records the measured addPieces call.

Batch	EVM-slot reads	EVM-slot writes	KAMT touched	KAMT modified	Newly occupied slots
1	10 → 12	7 → 4	11 → 7	6 → 3	5 → 2
4	21 → 25	28 → 13	24 → 7	19 → 3	20 → 8
16	69 → 85	112 → 49	72 → 8	67 → 4	80 → 32
32	133 → 165	224 → 97	137 → 9	132 → 5	160 → 64

Values are legacy → compact.

Findings

• Storage footprint: consistently 60% lower—5N → 2N newly occupied slots.
• EVM-slot writes: reduced from 7N to 3N + 1; reduction grows from 42.9% at one piece to 56.7% at 32 pieces.
• KAMT locality: compact contiguous records substantially reduce object activity as batches grow:
  • 4 pieces: 24 → 7 touched, 19 → 3 modified.
  • 16 pieces: 72 → 8 touched, 67 → 4 modified.
  • 32 pieces: 137 → 9 touched, 132 → 5 modified.
• EVM-slot reads: increase by 19.0–24.1%. These are repeated reads of already-accessed values, which are cached; they do not correspond to a comparable increase in KAMT object activity.
• Scaling: the compact layout modifies two shared objects plus roughly one compact-data object per 16 pieces, subject to alignment. The legacy representation modifies approximately four piece-data objects per piece.

At 32 pieces, the compact layout occupies 64 slots instead of 160, touches 9 KAMT objects instead of 137, and modifies 5 instead of 132.

Compatibility note

This is a forward-only prototype. Backwards compatibility for datasets created before the compact representation was not considered.

Keep this PR as draft; it is not ready to merge or deploy as an upgrade.

Verification

make test
209 passed, 0 failed