Add missing tests for KVIndex/randomts; fix three silent bugs

[Copilot]

Two source modules (KVIndex, pseudoRandomTimestamp) had no dedicated tests. While adding coverage, three bugs were found and fixed.

Bug fixes

• transaction.ts — SET validation never fired
  create() checked this.operation === KVOperation.SET but this.operation is undefined until after that line. The guard was effectively dead code.

  // before — always false, guard never triggered
  if (this.operation === KVOperation.SET && value === undefined) …
  // after
  if (operation === KVOperation.SET && value === undefined) …

• ledger.ts vacuum — missing await on readHeader()
  The comment read "update the header to catch new transactions" but the call was fire-and-forget, so the updated offset was never applied within the loop.

• ledger.ts lock — module-level LOCKED_BYTES constant mutated on every acquisition
  const lockBytes = LOCKED_BYTES + setBigUint64 wrote the lock-ID into the shared constant's underlying buffer. Replaced with new Uint8Array(LOCKED_BYTES_LENGTH) per call; removed the now-unused import.

New tests

• test/index.test.ts (22 tests) — KVIndex: add/get with sorting, limit, reverse, numeric & string ranges, empty ranges; delete including empty-parent cleanup and sibling preservation; clear; setIndex; getChildKeys.
• test/randomts.test.ts (6 tests) — pseudoRandomTimestamp: return type, proximity to original value, non-determinism, custom numBits, zero input, upper-bit preservation.
• test/transaction.test.ts — one test confirming the fixed SET/undefined guard now throws.

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