Add starter compatibility matrix e2e + version.ts floor guard

[demtario]

Summary

Task: https://app.clickup.com/t/9015210959/DEV-2102

• Adds runner/e2e/starter-matrix.spec.ts — opt-in e2e (E2E_STARTER_MATRIX=1) that empirically boots every starter in examples/ at every supported Handsontable major (15-19) against a live instance, per DEV-2102 / ADR-0021 decision 10.
• Adds runner/scripts/starter-matrix-report.mjs to turn the run's JSON into a markdown compatibility table (pnpm e2e:matrix:report, supports merging chunked runs).
• Adds DEFAULT_MIN_MAJOR = 15 floor to validateHandsontableVersion in packages/runtime/src/version.ts, mirroring the existing ceiling — previously only GET /api/versions enforced the 15 floor, so a direct session/API call bypassing the version dropdown could still request an untested major.
• Docs: runner/docs/starter-compat-matrix.md (how to run it, prod concurrency cap, limitations) + pointer in run-and-deploy.md.

Findings (docs/reports/starter-matrix-2026-07-17.md)

• 10/16 starters fully clean across majors 15-18 (react, react-js, vue, javascript, typescript, example1, next.js, nuxt, remix).
• 5 React-UI-lib container starters (ant-design, mui, base-web, fluent-ui, next-shadcn.js) break at majors 15-16 only — dev server boots but the grid never mounts. mui's console log points at an npm package-exports resolution error.
• angular breaks at majors 15-17, passes only at 18 (never reaches "Live" — container never finishes booting).
• astro fails at every tested major with an identical, version-independent 504 error — reads as an Astro-container infra issue, not a Handsontable compatibility problem.
• Major 19 has no stable npm release yet — skipped everywhere, not failed.
• Per-starter/per-major UI restriction is deliberately not done here (out of scope per task) — flagged as follow-up work informed by this report.

Test plan

• pnpm test (unit, includes new floor-guard cases) — 34/34 pass
• pnpm typecheck — clean
• pnpm e2e — deterministic suite unaffected (matrix all-skipped without the flag; pre-existing docs-examples failures confirmed unrelated via baseline comparison)
• Full pnpm e2e:matrix run against https://demos.handsontable.com (chunked), report generated and reviewed

🤖 Generated with Claude Code

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