nipcash, nipcw: client-side request checks, resend safety, and the remaining test gaps

[naliyi]

Closes #124

Summary

• Request builders refuse an empty mint_cash, a connection_key without ia_pubkey, a negative or sub-second expiry, duplicate consolidate sources and a nil transfer target (previously a panic).
• ItemOutcome.SafeToResend is false for a Hub success the client could not decode (new Undecodable flag).
• BatchSession's *Many methods refuse a bill minted by another node instead of sending it into silence.
• The private transport's "no relay reachable" errors name every relay tried.
• Adds tests for proof bindings, wire literals, decoder TLV rules, the proof freshness window and the kind-23199 builder; end-to-end tests now use the test deadline instead of fixed 20–30 s timeouts.

Test plan

• Each behavior fix fails its new test when the fix is removed; each test-only addition was checked against the mutation it guards (mutant red, reverted green).
• go vet, go mod tidy, gofmt, golangci-lint v2.13.2, govulncheck, go test -race -count=1 ./...
• nipcash/client race suite run three times concurrently on a loaded machine: green.
• Downstream against this branch: cashctl build/vet/tests, lokihub build/vet/tests, ncli build/vet + Decode tests.