Prefer absolute imports for source and tests

[crankshift]

Summary

Prefer absolute imports such as @/core/... over deep relative imports like ../../... across source and tests.

Context

Deep relative imports make files harder to move during refactors. The alias should work consistently for runtime, build tooling, tests, and linting before imports are updated.

Acceptance Criteria

• Runtime and package import alias support is configured for @/... imports.
• Vitest resolves the alias in tests.
• Biome support is configured or verified for the alias.
• Source imports are updated consistently where the alias improves readability.
• Test imports are updated consistently where the alias improves readability.
• Normal verification passes after the import migration.

Notes

Do not introduce broad compatibility layers unless the current build or runtime requires them.