feat(anatomist): add package classifying main-content layout from box geometry

[YusukeHirao]

Summary

• Adds @d-zero/anatomist, a new CLI/library package that renders a URL in a real browser and classifies its main-content child blocks into visual layout patterns (vertical stack, horizontal row, simple/complex grid, table, float-wrap) from measured box geometry (getBoundingClientRect), using CSS properties only as corroborating signal.
• Exports @d-zero/beholder's main-content selector priority/fallback lists (MAIN_CONTENT_SELECTORS / MAIN_CONTENT_FALLBACK_SELECTORS) as shared constants so anatomist's own main-element resolution reuses the exact same heuristic instead of duplicating it.

Details

• capture layer (capture-layout-tree.ts / capture-layout.ts): resolves the main-content element and walks its subtree into raw geometry + style + innerHTML via a single page.evaluate. Every helper is nested inside the evaluated function since Puppeteer only serializes that function's own source text into the browser realm.
• classify layer (pure functions, no browser dependency): row/column geometry clustering, per-pattern detectors (table/grid/horizontal-row/float-wrap/vertical-stack), a carousel-overflow guard, and single-child-wrapper collapsing so a main > div.inner > div.container chain doesn't end classification immediately after the root.
• orchestration (analyze-page-layout.ts / run-batch.ts): one URL across multiple viewports (PC → tablet → mobile, largest first) reusing one page; many URLs concurrently via @d-zero/dealer, each in its own tab of one shared browser.
• CLI (cli.ts): reads a URL list from stdin or --input, writes JSONL to stdout or --out.
• Fixture-driven integration tests (classify-layout-tree.integration.spec.ts) exercise capture → classify end-to-end against real HTML fixtures for each layout pattern, using @d-zero/beholder's actual selector constants.

Test plan

• yarn build — all 29 projects build successfully
• yarn test — 1854 tests pass across 142 files (including new fixture-driven integration tests)
• yarn lint — no errors
• /code-review medium, /qa-engineer, /product-manager reviewed; all findings addressed (see commit history for the fixes: closure-free page.evaluate bug, Math.max(...spread) RangeError risk, --concurrency 0 hang, duplicate row-clustering, dead code removal, bidirectional selector-sync test, missing --input/--out coverage)