docs(adr): record the layout-aware build model

[NWarila]

Summary

• Add ADR-template/0003 documenting the as-built layout-aware build model.
• Record how .github/.script and .github/.module markers are enforced, how canonical build.ps1 dispatches by layout, and why single-script and module builds intentionally differ.
• Capture deferred items around consumer-specific literals, push-based drift sync, and harden-runner adoption.

Impact

Docs-only. No code, CI, tests, or layout markers are changed.

Validation

• git diff --name-only origin/main..HEAD returns only docs/decision-records/template/0003-layout-aware-build.md.
• ADR mirrors the section structure and status table style of ADR-template/0001 and explicitly extends it.

Audit Note

Do not merge yet; this is ready for review by the audit step.