feat: emit compact PiecesAddedV2 events

[Kubuxu]

Closes #295

Summary

Adds a compact, ABI-decodable event for piece additions:

struct PackedCid {
    bytes32 header;
    bytes32 root;
}

event PiecesAddedV2(
    uint256 indexed setId,
    uint256 firstPieceId,
    PackedCid[] pieceCids
);

Piece IDs derive from firstPieceId + arrayIndex. CIDs are reconstructed by stripping the leading zero padding from header and appending root.

Events are limited to 125 pieces. Larger calls emit multiple events with adjusted firstPieceId values.

BREAKING (!!): event consumers must upgrade

The legacy PiecesAdded event remains in the contract ABI but is no longer emitted. New additions emit only PiecesAddedV2.

Indexers and all other event consumers must support PiecesAddedV2 before the contract rollout.

Historical PiecesAdded logs remain decodable. The addPieces function ABI and listener callbacks are unchanged.

Size

Encoding	64 pieces	Maximum pieces/event
Existing PiecesAdded	12,480 bytes	41
PiecesAddedV2	4,256 bytes	125

A 125-piece event consumes 8,160 bytes, below FEVM’s 8,192-byte limit.

Gas

Measured with this PR applied, using the prior batch sizes of 41 pieces with one metadata pair and 61 pieces without metadata as baselines.

Metadata per piece	Starting pieces	Prior batch: gas/piece	Largest batch: gas/piece	Change
One pair	0	41: 71.003M	135: 59.633M	−16.0%
One pair	1,024	41: 71.664M	135: 59.961M	−16.3%
None	0	61: 10.063M	225: 6.505M	−35.4%
None	1,024	61: 10.455M	225: 6.877M	−34.2%

Dependency

Relies on #292 enforcing canonical PieceCIDv2 encodings so splitting a CID into header and root preserves it exactly.

Validation

• Packed CID and 125/126-piece batching coverage
• Full suite: 224 passed
• Contract size checks pass