BQIP-0003: Wallet & ecosystem standards (PQC PSBT, addresses, SDK)

[AlphaB135]

Draft spec scaffold added in docs/spec/BQIP-0003.md.\n\nChecklist:\n- [ ] Finalize HRP/witness/hash params\n- [ ] Assign PSBT key types + encoding\n- [ ] SDK API + ref impl (Rust/TS)\n- [ ] Exchange integration guide + tests