Implement deterministic rule precedence resolution

[Matographo]

Summary

Apply exact-path over extension over global precedence, with later definitions winning at equal specificity.

Why

The engine must resolve configuration deterministically for every target file.

Acceptance Criteria

• Exact path overrides extension
• Extension overrides global
• Later rules win when specificity is equal
• Behavior is covered by tests

Dependencies

• Depends on Implement the rule model and parser for global rules #11
• Depends on Implement selector rules for file extensions and exact relative paths #12