Improve DomStack programmatic build testing support

[bcomnes]

This PR completes the programmatic API testing improvements tracked in #234.

Completed

• Resolves relative copy paths passed to the DomStack constructor to absolute paths from the current working directory, matching CLI --copy behavior.
• Stores resolved copy paths in this.opts.copy so downstream build and watch copy steps receive consistent absolute paths.
• Adds constructor copy-path tests for relative, absolute, and mixed copy paths.
• Documents programmatic copy path behavior in README.md and the DomStackOpts JSDoc.
• Improves worker page-build error reporting by restoring worker errors with page/template context in the error message, while preserving cause, stack, and attached metadata.
• Adds a regression assertion that page build errors include the affected page path.
• Adds a top-level testBuild(src, opts) helper for isolated programmatic test builds into temporary directories.
• Documents testBuild() usage and adds fixture coverage for generated output reading, copy-option pass-through, and cleanup.

Validation

• node --test --test-reporter spec test-cases/page-build-errors/index.test.js
• node --test --test-reporter spec test-cases/test-build-helper/index.test.js
• npm run test:tsc
• npm run test:neostandard
• npm run test:node-test
• npm run test:installed-check

Closes #234

[coveralls]

Coverage Report for CI Build 28271793017

Coverage decreased (-0.6%) to 91.837%

Details

• Coverage decreased (-0.6%) from the base build.
• Patch coverage: 7 uncovered changes across 1 file (88 of 95 lines covered, 92.63%).
• No coverage regressions found.

Uncovered Changes

File	Changed	Covered	%
lib/build-pages/index.js	50	43	86.0%
Total (3 files)	95	88	92.63%

Coverage Regressions

No coverage regressions found.

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Coverage Stats

Relevant Lines:	4254
Covered Lines:	4027
Line Coverage:	94.66%
Relevant Branches:	830
Covered Branches:	642
Branch Coverage:	77.35%
Branches in Coverage %:	Yes
Coverage Strength:	74.09 hits per line

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💛 - Coveralls

[Copilot]

Pull request overview

Fixes programmatic DomStack usage so relative copy paths are normalized consistently (stored as absolute paths) based on the src location rather than the process working directory.

Changes:

• Resolve opts.copy entries to absolute paths in the DomStack constructor.
• Persist resolved copy paths back onto this.opts.copy so downstream copy steps/watchers consume consistent absolute paths.
• Simplify dest-containment validation to operate on already-resolved copy paths.

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[Copilot]

Pull request overview

Copilot reviewed 1 out of 1 changed files in this pull request and generated no new comments.

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[Copilot]

Pull request overview

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

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[bcomnes]

This is probably fine but only partially imements all suggestions in the parent issue.

[bcomnes]

This only addresses the "The constructor does not resolve copy paths" issue. It also warrants a document README.md update and a jsdoc update for the public interface.

[bcomnes]

All issues addressed.