fix(scanner_ws_parse): flatten block arm so llvm-cov tracks line 37

[TaprootFreak]

Problem

PR #87 (event-driven scanner) merged into develop with a Coverage Gate FAILURE: server/src/scanner_ws_parse.rs:37 is reported as uncovered by llvm-cov despite parse_ws_frame_returns_empty_when_block_id_is_invalid_hex exercising that exact path in local cargo test.

Root cause: the nested if let Some(block) { if let Some(hash) { if let Ok(h) = BlockHash::from_str(hash) { ... } } } shape produces a closing-brace region at line 37 that llvm-cov reports as an untaken region, even though every reachable input either short-circuits earlier or returns on the inner success arm. This is an llvm-cov region-tracking quirk with nested if let, not a real coverage hole.

Consequence: every PR based on develop now inherits a red Coverage Gate. This is currently blocking:

• #18 (Release: develop → main, auto-generated)
• #90 (fix(server): prepare-then-commit mint to prevent state desync — closes Mint state-desync class: num_pubkeys advances past on-chain commitments after absorbed publisher errors #89)

Fix

Flatten the block arm to a single Option chain. Behavior is byte-identical across every input shape the existing 6 tests cover; only LLVM's region tracking changes (the closing-brace region at line 37 no longer exists as a distinct sub-region).

// before
if let Some(block) = value.get("block") {
    if let Some(hash) = block.get("id").and_then(|v| v.as_str()) {
        if let Ok(h) = BlockHash::from_str(hash) {
            return vec![h];
        }
    }
    return Vec::new();
}

// after
if let Some(block) = value.get("block") {
    return block
        .get("id")
        .and_then(|v| v.as_str())
        .and_then(|s| BlockHash::from_str(s).ok())
        .map(|h| vec![h])
        .unwrap_or_default();
}

The blocks arm and frame_signals_tx_seen are untouched. No new tests needed — the existing 6 tests cover every input shape (valid hex, invalid hex via "not-a-real-hash" and "zzzz", missing id, non-string id, array shape, malformed JSON, unknown shapes).

Test plan

• CI green (Lint & Build + Server + Shared Tests + Coverage Gate all pass).
• After merge: PR Release: develop -> main #18 and PR fix(server): prepare-then-commit mint to prevent state desync #90 re-run with Coverage Gate green (the line-37 region no longer exists).