perf(subgraph): cut per-event store round-trips

[silent-cipher]

Problem

Two guaranteed-miss Postgres lookups happen on hot paths:

1. Transaction dedup via Transaction.load() in every handler (handleDataSetCreated, handleDataSetDeleted, handleStorageProviderChanged, handlePossessionProven, handleNextProvingPeriod, handlePiecesAdded in subgraph/src/pdp-verifier.ts). A transaction's events are always processed within one block, so the entity can only pre-exist in the in-block cache — but load() on a not-yet-existing entity is a guaranteed DB round-trip for the first event of each tx.

2. Root.load() existence check in handlePiecesAdded (pdp-verifier.ts:1109-1116). The contract's nextPieceId is monotonic — piece IDs are never reused — and the code's own log message says "This shouldn't happen." That's one DB miss per piece added during backfill.

Proposal

• Replace Transaction.load(id) with Transaction.loadInBlock(id) at all sites (apiVersion is 0.0.9, so it's available). This is the documented use case for loadInBlock: entities created earlier in the same block.
• Drop the Root.load existence check and construct the Root unconditionally.

Acceptance criteria

• No Transaction.load() remains in pdp-verifier.ts
• No existence check before Root creation in handlePiecesAdded
• Tests pass (multi-event-per-tx fixtures still produce a single Transaction entity)