PDP pull fixups: addPieces nonce/payload/metadata + shovel null blocks

[parkan]

PDP pull fixes stranded on a working branch, never merged after #678. Companion to #688 (content-provider serving v2 piece CIDs) -- together they make the FWSS pull path work end to end.

• addPieces used a fixed zero nonce (FWSS keys replay on clientNonces[payer][nonce]) and nil metadata (rejected MetadataArrayCountMismatch); now a fresh nonce + per-piece metadata
• CommPv2 CID was derived from padded size * 127/128 instead of the real CAR bytes; PayloadSize (car.FileSize) is now plumbed through and encoded, with a guard
• manual sends set the start/end epoch sentinel; added --payload-size
• shovel pinned to a fork tolerating null blocks in the indexer (revert when upstreamed)