feat: add Zcash to explorer-metadata

[MatiasOS]

Summary

Add Zcash (mainnet + testnet) to the openscan-explorer/explorer-metadata repository so the explorer can load it the same way it loads Bitcoin and Solana.

Motivation

data/networks.json and the accompanying schema are the source of truth for which networks the explorer knows about. Zcash needs entries there before #398 can render anything. Filed here for tracking; the work itself lands in explorer-metadata.

Proposed Solution

1. schemas/network.schema.json — extend the type enum. It is currently ["evm", "bitcoin", "solana"]; add "zcash". This is the blocking change: without it the new entries fail schema validation.

Zcash is a Bitcoin fork and reuses the bip122: CAIP-2 namespace, so it is tempting to reuse type: "bitcoin". Don't — the RPC surface differs enough (shielded pools, hash_or_height as a string, numeric verbosity, object-bag address-index params) that the explorer needs its own adapter branch, which keys off type.

2. data/networks.json — add two entries, following the existing Bitcoin shape:

{
  "type": "zcash",
  "networkId": "bip122:00040fe8ec8471911baa1db1266ea15d",
  "slug": "zec",
  "name": "Zcash Mainnet",
  "shortName": "Zcash",
  "description": "Privacy-preserving cryptocurrency with shielded transactions",
  "currency": "ZEC",
  "color": "#F4B728",
  "isTestnet": false,
  "logo": "assets/networks/zcash-mainnet.svg",
  "links": [
    { "name": "Website", "url": "https://z.cash", "description": "Official Zcash website" },
    { "name": "Docs", "url": "https://zebra.zfnd.org", "description": "Zebra node documentation" },
    { "name": "GitHub", "url": "https://github.com/ZcashFoundation/zebra", "description": "Zebra repository" }
  ]
}

Plus a testnet entry with networkId: "bip122:05a60a92d99d85997cce3b87616c089f", slug: "tzec", isTestnet: true.

Both networkId values are CAIP-2 identifiers derived from the first 32 hex chars of each genesis block hash, verified live via getblockhash(0):

Network	Genesis hash	chain
Mainnet	00040fe8ec8471911baa1db1266ea15dd06b4a8a5c453883c000b031973dce08	main
Testnet	05a60a92d99d85997cce3b87616c089f6124d7342af37106edc76126334a2c38	test

Note Zcash is not in the chainagnostic bip122 registry (which lists only Bitcoin, Bitcoin Cash, Litecoin and Dogecoin), so these are derived from the CAIP-4 rule rather than copied — same as how the Bitcoin entries were produced.

3. data/rpcs/ — add a zec/ directory alongside the existing btc, evm, solana.

⚠️ RPC availability is genuinely poor, and this is the main risk on this issue. I probed every candidate provider: zcash.drpc.org returns Unknown network, blockpi returns unknown host, lava.build 403s, and PublicNode, Ankr, GetBlock, Blockdaemon, OnFinality, Dwellir, AllThatNode and BlastAPI have no Zcash host at all. The only free no-key JSON-RPC endpoints are two Tatum gateways:

https://zcash-mainnet-zebrad.gateway.tatum.io
https://zcash-mainnet.gateway.tatum.io

Measured constraints on those:

• They share a single per-IP bucket of 5 req/min — verified by alternating rapid-fire requests across both hosts (1–3 succeeded, 4–8 all 429, regardless of host). Listing both buys redundancy, not throughput.
• The gateway whitelists methods. Working: getblockchaininfo, getblockcount, getbestblockhash, getblockhash, getblock (verbosity 1 and 2), getblockheader, getrawtransaction, gettxout, getdifficulty, getmempoolinfo, getrawmempool, getnetworkinfo, validateaddress. Blocked with -32601: every z_* and getaddress* method, plus getinfo, getpeerinfo, getmininginfo, getblocksubsidy, getstandardfee, getbestblockheightandhash.
• An API key raises the rate limit but does not unlock any method — I tested a real key against all 14 blocked methods and every one still returned -32601. The whitelist is fixed, not tier-based.

So a hosted-only setup supports the core explorer views (blocks, transactions, mempool, address validation) but cannot support shielded-pool or address-index features. Those need a self-hosted zebrad (mainnet RPC port 8232, enable_cookie_auth = false). Worth deciding up front whether we run one.

4. assets/networks/ — add a Zcash logo (zcash-mainnet.svg, and testnet variant if that is the convention).

Acceptance Criteria

• "zcash" added to the type enum in schemas/network.schema.json
• Mainnet + testnet entries added to data/networks.json and passing schema validation
• data/rpcs/zec/ created with the reachable endpoints
• Zcash logo added under assets/networks/
• Decision recorded on whether we self-host zebrad for the shielded/address-index surface