RFP-003: add Zcash transparent pair, scope out shielded

[fryorcraken]

Summary

• Adds LEZ–ZEC as the fourth supported pair in RFP-003, scoped to Zcash transparent (`t1...`) addresses via BIP-199 HTLCs, with ECDSA adaptor signatures as an encouraged variant for cross-chain unlinkability.
• Adds a new appendix `appendix/zcash-atomic-swap-primitives.md` capturing the feasibility matrix, the re-randomization wrinkle that makes shielded adaptor swaps research-grade, and the trust-model distinction between atomic swaps and TSS-vault routing (Maya).
• Explicitly puts Zcash shielded (Sapling / Orchard) out of scope. Rationale: no production atomic-swap construction for shielded pools exists today, the cryptographic state of the art does not provide a drop-in design for RedJubjub / RedPallas with per-spend re-randomization, and treating shielded as research-grade matches what every production Zcash swap path (Zwap, Maya) actually does.

Updates: Functionality req #5 (Zcash transparent), Usability reqs (LEZ–ZEC SDK, `zcashd`/`zebrad` node setup), per-pair SDK doc-packet entries, demo checklist, Recommended Team Profile, and Resources.

Test plan

• Render `RFPs/RFP-003-atomic-swaps.md` and confirm the renumbered Functionality requirements (1–11) are sequential and consistent
• Confirm the new appendix renders and the relative link from RFP-003 → appendix resolves
• Sanity-check the Zcash external links (BIP-199, ZIP-202/203, Zwap thread, librustzcash)

Related

This is a separate, independent change from #48 (LEZ primitive status) — both can land independently.

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