Broaden e2e coverage: pin 12 core behaviors (issue #112)

[vchelaru]

Summary

Widens the Playwright .NET e2e suite from 2 smoke tests to 14 in one PR, pinning the ~12 most important user-visible behaviors so a regression in any core path fails CI (issue #112). Not aiming for 100% coverage — "confident the basics work."

Tests added (CoreBehaviorsTest.cs)

1. Compile error surfaces cleanly — no game starts, error banner stays hidden
2. Stop clears the running game (status "Stopped.", button reverts to "Run")
3. Edit-then-rerun: fresh compile vs cache-hit vs recompile (pins CompileFingerprint)
4. Load a different example from the modal → code loads + runs
5. Sample-switch A→B→A stays alive (full Mouse trail example broken on desktop (suspected #90/PR #93 regression) #95, asserted via the GetInputDebugState hook)
6. Reach↔HiDef profile switch flips webgl↔webgl2 (issue Shared canvas can't change WebGL context type — switching between Reach and HiDef games in one session crashes #25 canvas-swap path)
7. #code= Share URL round-trip (reload restores identical source + runs)
8. #snippet= Share URL round-trip (through SnippetReverter/SnippetExpander)
9. Cold-boot #code= deep link auto-runs (payload built with the real UrlCodec)
10. Monaco boots with default code visible
11. IntelliSense completion returns results (calls GetCompletionsAsync directly)
12. ?embed=true hides the editor + runs the game

Harness

• Extracted E2ETestBase: host + browser + one IBrowserContext shared in OneTimeSetUp; a fresh IPage per test so a running game / set _canvasContextType can't leak across tests (context shared so the WASM payload stays cached). SmokeTest now extends it.
• E2E project gains a ProjectReference to XnaFiddle.Core for deterministic UrlCodec payloads.

App-code changes — test hooks only, no behavior change

• data-testid on: Stop, Examples, Share, Assets buttons; example cards (+ data-example-name); the share URL input; snippet toggle; diagnostics pane; canvas.
• One debug hook: GetInputDebugState JSInvokable ({ gameRunning, mouseX, mouseY, touchCount }) for the sample-switch aliveness assertion — the one hook the e2e-testing skill's boundary allows.

Cuts (no silent drops)

• Automatic using statements (ctrl+dot, hover over?) #13 file-input asset load — the app has no file <input>; drag-drop can't be synthesized headless.
• Investigate performance issues #14 shader .fx compile — downloads ~17 MB DXC wasm, slow; keeps the e2e wall-clock sane (shader logic is unit-tested).
• Improve XnaFiddle on phone #15 export .zip — lowest value (can't validate the zip builds in e2e).

Notes

• Mouse trail example broken on desktop (suspected #90/PR #93 regression) #95 folded in (tests 4/5) and closed as superseded.
• CI stays windows-latest; dotnet test auto-discovers the new tests — no workflow edit.
• Local run: 14/14 green in ~1.5 min (no shader test).
• Skill e2e-testing updated: per-test isolation, the fragment-only-navigation no-reload gotcha, and the new test hooks.

Closes #112
Closes #95

🤖 Generated with Claude Code