Implement scope-aware import checking

[heartwilltell]

Phase 5: Compiler Integration — RFC #218

Enforce dependency scope rules at compile time. A regular .run file in a release build cannot import test-only or debug-only dependencies. Produce clear error messages indicating the scope violation.

Scope Rules (from RFC)

• _test.run files may import from all scopes
• Regular .run files in debug builds may import from dependencies, dev, and debug scopes
• Regular .run files in release builds may import from dependencies and dev scopes only

Acceptance Criteria

• Scope checking enforced based on file type and build mode
• Clear error: error: "profiler" is a debug-only dependency and cannot be imported in release builds
• Build mode (debug/release/test) threaded through compilation options
• Tests for each scope combination

Files

• Modified: src/resolve.zig

Dependencies

• Extend import resolution to recognize external module paths #312 (external import recognition)

Can be worked on in parallel with #314 (multi-file compilation).

Part of the Package Manager RFC.