fix: schema-graph drawer stale-write race, cancellation, progressive draw (#124)

[BorisTyshkevich]

What & why

Fixes #124: the inline schema-lineage graph (drag/click a db/table onto the
results pane) had a stale-write race, no cancellation, and no progressive
draw — a slow schema left the pane blank for the whole round trip with no way
to stop it.

• Stale-write race (same class as inbox: openNodeDetail/openDetailPane has no stale-response guard #97). showSchemaGraph now captures a
  request-identity token and checks it before every write, so a Run/Explain
  or a second graph request that replaces the tab's result mid-fetch can
  never have a superseded fetch's resolution land on the wrong result.
  Covered by regression tests reproducing the race for both run() and
  runScript(), plus a last-triggered-wins test for two rapid graph requests.
• Cancellation. app.state.schemaGraphAbortController + a
  cancelSchemaGraph({clearResult}) helper — auto-cancel on Run/Explain/a new
  graph request, and a manual Cancel button (mirrors the existing
  .exp-cancel export-progress pattern) on both the pre-draw placeholder and
  the in-progress toolbar. Cancelling keeps an already-drawn free-edges graph
  on screen (marked partial) or falls back to the empty-results placeholder,
  whichever has something to show. tryQueryData now rethrows a genuine
  cancellation (gated on the caller's own signal.aborted) instead of
  silently degrading system.dictionaries to "no dictionaries, continue".
• Progressive draw, gated by size. loadSchemaLineage fires an onBase
  callback with the free edges (dependencies/target/engine-arg/dictionary —
  no extra round trip) as soon as they're known, then a single second layout
  merges in the view/MV source edges once EXPLAIN AST settles, with a
  "resolving N/M…" toolbar readout. Below AST_PROGRESSIVE_THRESHOLD (50)
  view/MV objects the two-phase split is skipped entirely — the whole fetch
  settles fast enough on a small schema that a visible first paint is just
  flicker, not a win.
• New trigger: expanding a closed db row in the tree now also draws its
  schema graph (matching drag-to-drawer); collapsing again doesn't re-fetch
  or re-draw.

Verified live against a real ClickHouse server (see PR description checklist
below) — progressive draw, the race fix, and both cancel paths all confirmed
end-to-end, including a tab-switch probe.

Checklist

• npm test passes (the per-file coverage gate is non-negotiable)
• Tests added/updated in the same change as the code
• npm run build succeeds (single-file dist/sql.html)
• Layers kept honest: pure logic in src/core/, network in src/net/ (injected fetch), DOM in src/ui/
• No new runtime dependency
• CHANGELOG.md ([Unreleased]) updated
• Reconciled affected tracked work — Schema-graph drawer: fix stale-write race, add cancellation, progressive draw #124 isn't a roadmap (Roadmap to 1.0.0 #68) checklist line item, no ADR change; CHANGELOG updated

Co-Authored-By: Claude Sonnet 5 noreply@anthropic.com
https://claude.ai/code/session_01784dKCpk5W7rdpUAwcQnix