Implement selector rules for file extensions and exact relative paths

[Matographo]

Summary

Support extension selectors and exact relative path selectors for rules.

Why

File-scoped rendering behavior depends on deterministic selector parsing before precedence can work.

Acceptance Criteria

• File-extension and exact-path selectors are supported
• Invalid selector syntax fails cleanly
• Selector records integrate with rule resolution

Dependencies

• Depends on Implement the rule model and parser for global rules #11