Refactor: deduplicate isCashAddr into shared btc.HasCashAddrPrefix

[Copilot]

The isCashAddr function was copy-pasted identically into both bchain/coins/bch and bchain/coins/ecash, differing only in the package-level prefix constants each referenced.

Changes

• bchain/coins/btc/bitcoinlikeparser.go — Added shared utility HasCashAddrPrefix(addr string, prefixes ...string) bool; both bch and ecash already import this package
• bchain/coins/bch/bcashparser.go — Removed isCashAddr, updated call site to btc.HasCashAddrPrefix(address, MainNetPrefix, TestNetPrefix, RegTestPrefix)
• bchain/coins/ecash/ecashparser.go — Same as above

// Before (duplicated in both packages):
func isCashAddr(addr string) bool {
    n := len(addr)
    switch {
    case n > len(MainNetPrefix) && addr[0:len(MainNetPrefix)] == MainNetPrefix:
        return true
    // ...
    }
    return false
}

// After (single shared function in btc package):
func HasCashAddrPrefix(addr string, prefixes ...string) bool {
    n := len(addr)
    for _, prefix := range prefixes {
        if n > len(prefix) && addr[0:len(prefix)] == prefix {
            return true
        }
    }
    return false
}

Warning

Firewall rules blocked me from connecting to one or more addresses (expand for details)

I tried to connect to the following addresses, but was blocked by firewall rules:

• dl.google.com
  • Triggering command: /usr/bin/wget wget REDACTED (dns block)

If you need me to access, download, or install something from one of these locations, you can either:

• Configure Actions setup steps to set up my environment, which run before the firewall is enabled
• Add the appropriate URLs or hosts to the custom allowlist in this repository's Copilot coding agent settings (admins only)

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