test(e2e): add WebKit/Safari to the Playwright matrix + decide the support stance (#69)

[BorisTyshkevich]

What & why

Adds WebKit as a third Playwright e2e engine and makes the explicit
supported-browser decision #69 asks for.

Why it matters: the whole layout and the pointer/caret/drag math ride on
html { zoom: var(--zoom) }, and WebKit is the engine most likely to diverge on
zoom × getBoundingClientRect/viewport units — yet it was the one engine the
e2e suite never exercised. This is Phase 0 of the roadmap (#68): a regression net
that lands before the CM6 (#21) and schema-graph (#66) refactors churn the
coordinate paths.

The empirical result (the deliverable): WebKit launches only on the Linux CI
runners (the frozen mac14-arm64 build SIGTRAPs under the dev sandbox — as do
chromium/firefox locally; these have always been Linux-CI-only), so CI is the
source of truth. On CI the suite is 36 passed = 12 specs × {Chromium,
Firefox, WebKit}. The WebKit-included specs — editor highlight/selection
alignment (the html{zoom} × scrollbar-shrink math), the schema-double-click
execCommand insertion path, the schema-lineage graph, and the EXPLAIN-pipeline
pan/zoom — all pass on WebKit. Run:
https://github.com/Altinity/altinity-sql-browser/actions/runs/28440418800

Decision: Safari (WebKit) is supported. The html{zoom} reliance does not
break Safari for the tested surfaces, and a future regression that would break
it now fails CI instead of shipping silently.

Changes

• playwright.config.js — add a webkit project; update the setup comments.
• .github/workflows/ci.yml — playwright install --with-deps … webkit and run
  it in the e2e job.
• README.md — new Supported browsers section recording the decision;
  three-engine e2e section + install hint; defers the full
  browser/ClickHouse/IdP matrix and the narrow-viewport stance to Document the supported-browser matrix (browsers, ClickHouse versions, IdP requirements) #71.
• CHANGELOG.md [Unreleased] — note the WebKit coverage + the stance.

Reconcile

• Browser row fed into the support-matrix issue Document the supported-browser matrix (browsers, ClickHouse versions, IdP requirements) #71 (its acceptance: "Browser
  row reflects the WebKit/Safari decision").
• Decision + CI evidence recorded on Add WebKit/Safari to e2e + decide the supported-browser stance #69.
• Roadmap Roadmap to 1.0.0 #68 Phase-0 / release-blocker checkbox for Add WebKit/Safari to e2e + decide the supported-browser stance #69 ticks on merge.

Closes #69.

Checklist

• npm test passes (the per-file coverage gate is non-negotiable) — no src/ change; gate green
• Tests added/updated in the same change as the code — the e2e "test" here is the new third engine; 36 passed on CI
• npm run build succeeds (single-file dist/sql.html) — 451513 bytes
• Layers kept honest: pure logic in src/core/, network in src/net/ (injected fetch), DOM in src/ui/ — no source touched (CI/test config + docs only)
• No new runtime dependency (@playwright/test is already a devDependency; no bundled-runtime change)
• README / CHANGELOG.md ([Unreleased]) updated

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