Migrate successful build fixtures to testBuild

[bcomnes]

Stacked on #251.

Summary

Migrates successful one-shot build fixtures to use the new top-level testBuild() helper introduced in #251.

Updated tests:

• test-cases/default-layout/index.test.js
• test-cases/drafts/index.test.js
• test-cases/general-features/index.test.js

These tests now build into isolated temporary destinations and register build.cleanup() with t.after().

Tests intentionally left on direct DomStack usage:

• error-path fixtures, because testBuild() throws before returning cleanup on failing builds
• nested-dest, because it specifically verifies building into a nested destination under the source tree
• watch tests, because they need direct DomStack.watch() control and stable temp source/dest paths
• constructor/type smoke tests, because they are not one-shot successful build fixtures

Validation

• node --test --test-reporter spec test-cases/default-layout/index.test.js test-cases/drafts/index.test.js test-cases/general-features/index.test.js
• npm run test:tsc
• npm run test:neostandard
• npm run test:node-test

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[coveralls]

Coverage Report for CI Build 28271434076

Coverage increased (+0.6%) to 92.55%

Details

• Coverage increased (+0.6%) from the base build.
• Patch coverage: No coverable lines changed in this PR.
• No coverage regressions found.

Uncovered Changes

No uncovered changes found.

Coverage Regressions

No coverage regressions found.

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Coverage Stats

Relevant Lines:	4254
Covered Lines:	4027
Line Coverage:	94.66%
Relevant Branches:	699
Covered Branches:	557
Branch Coverage:	79.69%
Branches in Coverage %:	Yes
Coverage Strength:	74.34 hits per line

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💛 - Coveralls