test(host): script specs start from a sanitized environment (gh-#776)

[genwave-radio]

Closes #776

Stacked on #780 (gh-#770) and #781 (gh-#775); the first three commits are theirs. Only the three test(host) commits are new here.

What

• New tests/GenWave.Host.Tests/Support/ScriptProcess.cs: the one way a Host spec starts launch.sh, build.sh, setup.sh, migrate.sh or tools/preflight.sh. Run strips every GW_*, SKIP_*, COMPOSE_*, BUILD, CONFIG, ADMIN_PASSWORD, MEDIA_DIR and the five Postgres/Icecast password variables before the child starts, sets PATH to the scratch bin dir alone, then applies GW_ENV_FILE and the scenario's own extraEnv. Start returns the live process (optional stdin redirect) for the two wizard facts that answer prompts. Sanitize covers the one setsid fact that must build its own start info.
• 18 spec files migrated onto the helper; their file-local RunScript/MakeBinDir/AddStub/RepoRoot copies are gone. No behaviour change: same env vars, same fixtures, same assertions.

Specs

• New Story438_ScriptEnvIsolation: red on main (helper missing), green here. Its grep fact holds: grep -rn 'new ProcessStartInfo("bash")' tests/GenWave.Host.Tests/Specs/ is empty.
• Full solution green with a clean parent shell (Host 2901 passed / 0 failed / 57 skipped), and identical with SKIP_PREFLIGHT=1 COMPOSE_FILE=/nope/compose.yaml GW_SPEC_CANARY_438=leak POSTGRES_PASSWORD=leak exported in the parent.

Wire check on the dev box (T475)

SKIP_PREFLIGHT=1 dotnet test tests/GenWave.Host.Tests --filter "FullyQualifiedName~FeatureScriptPreflight|FullyQualifiedName~FeaturePreflightExpansion|FullyQualifiedName~FeatureScriptSpecsDoNotInheritTheDevelopersShell":

Passed!  - Failed:     0, Passed:    66, Skipped:     0, Total:    66, Duration: 351 ms - GenWave.Host.Tests.dll (net10.0)

SKIP_PREFLIGHT=1 SKIP_TESTS=0 ./build.sh:

==> GenWave build (config: Release, version: v5.8.2-9-g133d75b)
==> dotnet build GenWave.sln
==> dotnet test
Passed!  - Failed:     0, Passed:    78, Skipped:     0, Total:    78 - GenWave.Context.Tests.dll (net10.0)
Passed!  - Failed:     0, Passed:   182, Skipped:     0, Total:   182 - GenWave.Core.Tests.dll (net10.0)
Passed!  - Failed:     0, Passed:   220, Skipped:     3, Total:   223 - GenWave.Ads.Tests.dll (net10.0)
Passed!  - Failed:     0, Passed:   437, Skipped:     3, Total:   440 - GenWave.Orchestration.Tests.dll (net10.0)
Passed!  - Failed:     0, Passed:    53, Skipped:     0, Total:    53 - GenWave.Plugins.Tests.dll (net10.0)
Passed!  - Failed:     0, Passed:   881, Skipped:    13, Total:   894 - GenWave.Tts.Tests.dll (net10.0)
Passed!  - Failed:     0, Passed:   146, Skipped:     0, Total:   146 - GenWave.Architecture.Tests.dll (net10.0)
Failed!  - Failed:   226, Passed:  2903, Skipped:   369, Total:  3498 - GenWave.Host.Tests.dll (net10.0)
Passed!  - Failed:     0, Passed:  1347, Skipped:    47, Total:  1394 - GenWave.MediaLibrary.Tests.dll (net10.0)
exit=1

The bypass itself is harmless: every preflight and script spec passed inside that child, which is the point of the env -u SKIP_PREFLIGHT line. The 226 Host failures are all Category=Integration Postgres classes and share one error, all predefined address pools have been fully subnetted. build.sh runs the suite unfiltered and with default xUnit parallelism, so on the dev box the per-class compose projects exhaust Docker's default address pools. CI runs --filter "Category!=Integration" and does not hit this. Pre-existing and unrelated to this branch; noted as a follow-up below.

Follow-ups (not this PR)

• build.sh test step: mirror CI's Category!=Integration filter or cap xUnit.MaxParallelThreads, so a dev-box build with tests enabled can go green.
• Story344/Story345 RunSetup still spool answers through a wrapper script; ScriptProcess.Start(redirectStdin: true) can replace that in a later pass.
• Two stale test containers from an earlier session (genwave-t386-…, genwave-t414-…) are still up on the dev box.