feat(stdlib): add seeded random service

[julia-script]

Summary

• add a portable, provider-replaceable Random service implemented entirely in Silk
• add deterministic SplitMix64-seeded Xoshiro256** plus nextBool, unbiased below, and allocation-free fillBytes
• ship the canonical source, manifest/generated artifacts, reference docs, OpenSpec artifacts, and cross-engine coverage
• align the LSP hover expectation with the compact documentation-section rendering

The API is explicitly deterministic and non-cryptographic; OS entropy remains out of scope.

Validation

• pnpm exec openspec validate add-seeded-random-service --strict
• pnpm typecheck
• pnpm exec biome check .
• pnpm test
• pnpm check
• pnpm release:candidate