Pipeline step library: enrich-merge (join a forEach result back onto its records) (child of #94)

[serge-ivo]

Gap found by the #94 capstone ("GAP #3" in lead-finder.test.ts).

Problem. The epic proof filter is "keep no-website OR unreachable." The "unreachable" half needs a reachability probe per place, then that {ok,code} merged back ONTO the place so filter can test it. Today http_reachable via forEach binds a parallel array [{ok,code},…] that sits beside the places with no way to correlate them: map/filter/dedupe_upsert each read a single items array, and there is no zip/join step. So only the "no-website" half of the filter is expressible; "unreachable" isn't.

Evidence. In the capstone the reachability array and the places array are the same length but nothing joins them by index; the shipped JSON filters on websiteUri missing only. Asserted as GAP #3.

Proposed fix (pick one):

1. An enrich step: {items:$ref, tool:"http_reachable", input:{url:{$item:"websiteUri"}}, as:"reachable"} — runs the tool per item (concurrency-capped, like fan_out) and writes each result under as on the item. This is the general "call a connector per record and merge" primitive the epic's enrich stage wants (also covers [connectors] Web-search connector (Google Custom Search / SerpAPI) — enrichment #99 web_search→extract_contacts for socials/email).
2. A zip/merge_by_index step joining two bound arrays.

Option 1 (a real enrich step) is the declarative primitive the epic's enrich bullet implies; it also depends on #94-child "dotted item access" for {$item:…} ergonomics. Part of #94.