fix(schema): clear empty-state for a DB with no lineage (was invisible/empty output)

[BorisTyshkevich]

Bug (reported)

Dragging database target_all on github.demo (201 tables, all URL engine, zero relationships) produced empty output.

Diagnosed live: buildSchemaGraph returned 201 nodes / 0 edges; dagre laid them out in a ~127621px × 58px single row; fit-to-view shrank every node to sub-pixel → a blank-looking pane.

Root cause

buildSchemaGraph pruned isolated nodes only inside else if (edges.length), so a DB with no relationships skipped pruning and dumped every table as a disconnected node. The only empty-state placeholder was keyed on nodeCount === 0 — unreachable for any real DB (≥1 table ⇒ ≥1 node) — so the degenerate case fell through to the unreadable strip.

Fixes (from a high-effort /code-review of the schema-graph feature, #41)

The bug

• schema-graph.js — whole-DB lineage now always keeps only connected (degree>0) nodes. A lineage view shows lineage; a relationless DB yields an empty graph.
• explain-graph.js — informative empty-state via schemaEmptyMessage: "No object relationships in <db> — its N tables aren't linked by a view, materialized view, dictionary, or Distributed/Buffer/Merge engine." (table focus: "<db.table> has no lineage relationships."). app.js carries tableCount.
• results.js — hide Expand when there's nothing to draw.
• attachPanZoom — minW = min(width/8, 600) so a wide graph can still be zoomed to a legible scale.

Also surfaced by the review

• ch-client.js — loadSchemaLineage used a local SQL quoter that dropped backslash-escaping; now uses core/format.js sqlString.
• schema-graph.js — index system.dictionaries by db.name (was O(D) find per dict); removed the dead void uuid.
• explain-graph.js — single placeholder() helper (was triplicated markup).
• README + tests — the doc promised unconditional pruning and a test ("keeps all tables when a DB has no lineage") certified the buggy behavior; both corrected.

Tests

New: no-relationship DB (50 URL tables → empty graph), the db/table empty-state messages, hidden Expand. 883 tests pass; per-file coverage gate holds.

Verified live

github.demo target_all now shows the message (not a strip); no Expand button. Deployed to otel / antalya / github.demo (sha 867ea3a).

Deferred (noted, not in this PR)

• A node cap / "graph is large — open fullscreen" guard for a genuinely large connected lineage (this PR resolves the degenerate edgeless case).
• Merge-engine O(K×N) scan and conditional fetch of create_table_query/as_select (payload) — minor at current scale.
• EXPLAIN-AST fan-out cap (already tracked in Schema lineage: cap the EXPLAIN AST fan-out on large schemas #42).

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