perf(api): the epistemic snapshot is quadratic and re-reads six ledgers on every call #1410

Problem

Two compounding problems in one function, found independently by two readers.

epistemic_snapshot in crates/graphforge-api/src/epistemic_snapshot.rs:

1. Is O(assertions times ledger sizes) on every read.
2. Separately re-reads six full ledgers from disk on every call.

It runs on read-only paths, including belief-projection and valid-time queries.

Acceptance criteria

• The snapshot does not re-read ledgers already read in the same operation.
• The quadratic term is removed or justified in writing.
• Measured: time and bytes read per call, before and after, against two project sizes with differing assertion counts.
• Epistemic semantics, belief projection and valid-time results unchanged.

Relationships

Found by the second redundancy sweep under #1387's directive. Third of eleven findings by frequency times cost.