[BLCCoreStudio]

Summary

Adds a deterministic discovery report for reviewed modules that have progressed beyond overview maturity and wires it into both CI discovery artifacts and the trusted public discovery publisher.

Changes:

• add scripts/build_depth_discovery.py
• emit depth.json and depth.md with guides and deep dives only
• derive review dates from the existing maturity manifest rather than inventing a second source of truth
• keep overview modules out of the focused depth view while explicitly stating that overview is a narrower scope, not a lower-quality state
• include deterministic kind/domain facet counts and direct module links so reviewed depth can be browsed by technology type and area
• add unit coverage for filtering, review metadata, Markdown/JSON output, determinism, public Markdown output, and facet navigation
• build the depth artifacts in Build Search Index
• surface reviewed guide/deep-dive counts and the generated report in the Actions summary
• include dist/depth in the existing discovery artifact upload
• have the trusted public-index workflow generate docs/depth.md from the same deterministic renderer and publish it only when discovery views actually change
• surface reviewed depth from the README and document it as a first-class discovery surface in docs/SEARCH.md
• document the maturity source-of-truth, local build commands, generated outputs, CI/publication behavior, and editorial boundary in docs/DEPTH_DISCOVERY.md

This is a focused discovery-layer step toward the roadmap item for better discovery of deep-dive modules. It does not change maturity classification, catalog data, search ranking, module content, protected curated content, or release behavior.