Implement TC39 SeededPRNG

[frostney]

Summary

Implement the Stage 2 TC39 proposal: https://github.com/tc39/proposal-seeded-random

Implement current Random and Random.Seeded APIs using proposal-specified ChaCha12, byte seed/state formats, random, seed, and save/restore semantics.

Why

This proposal fits GocciaScript's modern ECMAScript surface and the 0.13.0 proposal roadmap. It should extend the engine's existing architecture rather than arrive as a userland shim.

Current behavior

The host has private SplitMix64 deterministic randomness, but JavaScript has no Random namespace or reproducible generator objects.

Expected behavior

Implement current Random and Random.Seeded APIs using proposal-specified ChaCha12, byte seed/state formats, random, seed, and save/restore semantics.

Scope notes

Do not expose SplitMix64 as the proposal algorithm. Keep host and script PRNGs separate, copy caller buffers, version/test persisted state, and verify identical vectors across platforms/realms. Additive API: no flag.

Related bytecode roadmap: #871 and #875. Update relevant documentation and the Goccia proposal inventory. Use the current proposal specification at implementation time; add focused project tests and matching pinned Test262/proposal coverage without regressions.

Acceptance criteria

• Parser/API surface, descriptors, evaluation order, errors, and protocol behavior match the current proposal.
• Implementation uses the identified shared engine seam without a duplicate compatibility shim.
• Focused positive, negative, abrupt-completion, and edge-case tests cover the proposal.
• Relevant project suites, pinned Test262 cases, formatting, and documentation checks pass.