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feat: make PCGState JSON-serializable (#179) (#330)

* feat: make PCGState JSON-serializable

Refactor PCGState so it contains only values that survive JSON.stringify /
JSON.parse. This unblocks use cases like local-first apps that need to
persist or sync RNG state across clients (issue #179).

The previous shape stored a getOutput function and four bigints (state,
streamId, algorithm.multiplier, algorithm.increment) directly on the
state object. Functions can't be serialized at all, and bigints are not
JSON-native.

Changes:
- Represent 64-bit values as { hi, lo } pairs of unsigned 32-bit numbers
  (Uint64), which fit comfortably inside JSON.
- Move multiplier, increment, and the output function table out of state
  into a module-level config registry keyed by a `variant` tag stored on
  the state. The only variant today is 'pcg32'.
- Replace the per-state getOutput closure with a getOutput(pcg) helper
  that looks up the function from the variant config.
- Expose toBigInt / fromBigInt helpers for callers that need to inspect
  the underlying 64-bit values.

The internal math still uses bigints; conversion happens at the edges of
each step. This is a breaking change to the PCGState shape.

* refactor: inline single-use bigint temporaries

Drop streamIdBig, two stateBig temporaries, the config local inside
getOutput, and the initial local inside createPcg's factory — each was
referenced exactly once. Keeps the hot-loop outputFn lookup in
randomIntImpl hoisted, since it's executed per iteration.

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Co-authored-by: Claude <noreply@anthropic.com>

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