feat: emit PiecesScheduledForRemoval when scheduling piece deletions

[nishant-uxs]

Summary

• Emit PiecesScheduledForRemoval(setId, pieceIds) from schedulePieceDeletions so indexers can observe scheduled removals immediately, without waiting for PiecesRemoved at the next proving boundary.
• Declare the event in IPDPEvents alongside the existing piece lifecycle events.

Closes #281

Files Changed

• src/interfaces/IPDPEvents.sol
• src/PDPVerifier.sol
• test/PDPVerifier.t.sol

Testing Performed

• forge build --via-ir
• forge test --via-ir (187 passed)

Known Limitations

• Does not change listener/piecesScheduledRemove behavior; event is for log-driven consumers.

Risk Assessment

• Low: additive event only; no storage layout or ABI-breaking logic changes beyond a new event topic.

[wjmelements]

emit after listener instead of before, in case listener rejects

[nishant-uxs]

Addressed in 8aba46b — emit now happens after the listener call, matching the other listener/emit sequences in PDPVerifier (e.g. _createDataSet, deleteDataSet, nextProvingPeriod), so a rejecting listener reverts before the event is recorded.