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 100 pieces. Larger calls emit multiple events with adjusted firstPieceId values. This is a conservative round-number cap below the theoretical 125-piece FEVM limit.

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.
Tracked in #302

Size

Encoding	64 pieces	Configured pieces/event
Existing PiecesAdded	12,480 bytes	41
PiecesAddedV2	4,256 bytes	100

A PiecesAddedV2 event consumes 160 + 64n bytes: two 32-byte topics, three 32-byte ABI words, and two 32-byte words per packed CID. A 100-piece event consumes 6,560 bytes. The theoretical maximum is 125 pieces (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 100/101-piece batching coverage
• Full suite: 232 passed
• Contract size checks pass

[wjmelements]

I suspect this will be removed from the ABI if it's unused. If so, might as well remove it from here now.

[Kubuxu]

I don't want to remove it from ABI as indexers still have to support handling it.
I would be ok removing it in the next version after the switchover is complete, but indexers will still have to understand it.

[Kubuxu]

Solidity includes explicitly declared events in the ABI even when they are not emitted. forge inspect confirms PiecesAdded remains in both ABIs. We intentionally retain it so the current published ABI remains sufficient to decode historical logs; removal can happen after the consumer migration.

[wjmelements]

how was this calculated?

[Kubuxu]

The limit is 125, based on the calculated event size, event size limit and running in ref-fvm. 100 was requested by Rod as just a round number.