test(router/lnurl): cover localhost branch of lnurlp_handler

[TaprootFreak]

Problem

The Coverage Gate on PR #169 (Promote staging→develop) failed at 3265/3266 lines = 99.97 % with one uncovered line:

node/src/router.rs: 2647

Source:

let scheme = if host.contains("localhost") {
    "http"        // ← line 2647, uncovered
} else {
    "https"
};

The https:// arm is already pinned by lnurlp_known_address_returns_pay_request (which sends Host: api.zkcoins.app). The http:// arm was not exercised — the workflow's --all-features flag in the consolidated CI (PR #167) brought the lnurl feature into the gate's scope and surfaced the gap.

Fix

New test lnurlp_localhost_host_returns_http_callback (node/src/router_tests.rs, alongside the existing lnurlp tests):

• Issues the same /.well-known/lnurlp/<prefix> request the existing happy-path test does.
• Sets Host: localhost:8080.
• Asserts the returned LNURL callback URL starts with http://localhost:8080/.

No production-code change, no schema change, no API-shape change.

Verification

Local (CI-equivalent invocations):

• cargo fmt --all -- --check → clean
• cargo clippy -p node -p shared -- -D warnings → clean
• cargo clippy -p node --all-features -- -D warnings → clean
• cargo check -p node --all-features → clean

Tests not run locally per feedback_zkcoins_node_ci_tests.md — CI will run the full suite.

Context: why this surfaced now

PR #169's CI ran on the new dfxai-1..5 runner pool (set up earlier today on dfxai, OrbStack, dedicated to zk-coins/node CI — see DFXServer/server commit 4347a4a). The dfxai pool ran 399/399 tests green in 48.5 min with 0 PoolTimedOut flakes — the dfx01 pool's recurring DB-test flake is gone in the new environment. The only remaining gate failure was this 1-line coverage gap, which is repo-scope and not infra-related.

Unblocks

PR #169 (Promote staging→develop), which in turn unblocks PR #166 (Release develop→main).