Reduce Iseberg CI to a fast 220-test core

[Marc-André Moreau (mamoreau-devolutions)]

Summary

• Replace the exhaustive Iseberg CI run with a shared scripts/Test-IsebergCore.ps1 runner on Windows, Linux, and macOS.
• Keep 220 of 1,104 Iseberg cases from the referenced Windows run: 80.1% fewer. Preserve inexpensive editor, installation, handshake, protocol, and subprocess checks plus representative execution, native input, cancellation, recovery, profile, and settings-editor workflows.
• Exclude repeated hosted-workbench, theme, and native-input permutations, and avoid rerunning unrelated terminal tests already covered by the terminal-only workflow. The complete regression suite remains available locally; no tests are deleted.
• Preserve sequential test hosts, TRX artifacts, hang diagnostics, and terminal-only Windows NativeAOT publish checks. Reduce the test-step timeout from 120 to 20 minutes and fail if a project filter executes no tests.

Evidence

The original Windows run spent approximately 65 minutes in the test step. Its retained core cases account for approximately 162 seconds of recorded test durations, versus approximately 62 minutes in the original UI suite alone. This is a historical projection, not a new hosted-run measurement.

Executed the exact new CI runner locally on Windows with the pinned PowerShell 7.6.6 runtime:

. .\scripts\Install-IsebergTestPowerShell.ps1
.\scripts\Test-IsebergCore.ps1

Passed: 220, Failed: 0, Skipped: 0, in 43 seconds including incremental builds (the first run took 56 seconds).

Project	Core cases
App	64
UI	132
Settings	11
Settings.Editor	13

Verified every executed test name against the intended selection in the original TRX results. Workflow YAML, path triggers, platform matrix, runner, timeout, and git diff --check also passed. Linux/macOS execution remains for CI.