feat(viewer): push-time panes-density readability lint

[ivanmkc]

What

A structurally-valid view can still render unreadably small when too much heavy content is stacked into one layout — the screwbits trigger case (rows, 5 panes, 4 heavy: markdown→flow→component→vegalite→component), where each pane gets a sliver of the viewport. Nothing caught this.

This adds a static, push-time, warn-only density lint that covers every push path (CLI, ADK listener, raw POST /push).

How it landed (adapted from the approved spec)

The approved design predates the severity-tagged geometry-findings system that now exists (geometryReport → Finding{severity,code,message}; server returns 200 + {warnings, findings} / 204 / 422 under x-termchart-strict; CLI prints findings). That system already provides all the plumbing the spec proposed as new — so rather than build a parallel readability.ts module + new response shape + CLI change, the density checks are folded into geometryReport and emitted as Findings. This also honors the spec's own principle: "single source of truth … no duplicated lint logic across packages."

• No server.ts change (already calls geometryReport and returns findings)
• No push.ts change (CLI already surfaces findings via printGeometry)
• New warn-only finding codes: rows-overstuffed, columns-too-many, grid-too-many, panes-nested, flow-too-many-nodes

Thresholds (recalibrated against the real corpus)

The spec called its numbers "starting points, not load-bearing constants." The rows rule of ">4 weight OR ≥3 heavy" flagged four curated gallery examples (c4-architecture: wt9/3-heavy; the three explainers: wt7/3-heavy), so it was tightened:

layout	warns when	screwbits	densest gallery
rows	≥4 heavy OR weight >10	wt10 / 4 heavy → warns	wt9 / 3 heavy → clean
columns	>3 panes	—	max 3 → clean
grid	>8 tiles	—	max 4 → clean
nested panes	any nested panes	—	none → clean
standalone flow	>30 nodes	—	max 14 → clean

A "panes density" calibration guard test asserts no shipped example trips a finding, keeping the thresholds honest.

Tests / verification

• Viewer typecheck (tsc) — 0 errors
• Full viewer suite — 478 pass (incl. 73 server/push tests + new calibration guards)
• CLI suite — 161 pass
• Real screwbits demo file → rows-overstuffed through the exact geometryReport the server calls

Rollout

Lint runs in the viewer server, so the deployed Cloud Run viewer must be redeployed for cloud pushes to surface these warnings; a local dev viewer picks it up immediately. Warn-only — never blocks a push.