Write 'PRNG System Contract' (evm/hedera-services/system-contracts/prng)

**Page:** `evm/hedera-services/system-contracts/prng.mdx`

**Context:** Siblings `hts`, `exchange-rate`, `account-service`, `schedule-service` are written; PRNG is the gap.

**Scope:** PRNG system contract (`0x169`) — `getPseudorandomSeed`, usage from Solidity, determinism/verifiability notes, sample contract. Cross-link `native/tutorials/advanced/random-number`.

### Acceptance criteria
- [ ] Replace the placeholder template with real content
- [ ] Remove `hidden: true` from the frontmatter
- [ ] Follow the style guide: **markdown tables only** (no HTML ``), and avoid AI-tell phrasing (no em-dashes, "delve", "leverage", "not just X but Y")
- [ ] Add working cross-links to related pages
- [ ] Confirm any code samples run on testnet