Refactor src/manifest.rs: Split module to satisfy ≤400-line rule

[coderabbitai]

Overview

The src/manifest.rs module currently exceeds the project's ≤400-line rule and needs to be decomposed into smaller, focused submodules to improve code organization and cohesion.

Proposed Changes

Suggested submodule decomposition:

• manifest/diagnostics.rs - Error handling and diagnostic types
• manifest/glob.rs - Glob pattern processing and path utilities
• manifest/expand.rs - Manifest expansion logic
• manifest/render.rs - Template rendering functionality

Keep in src/manifest.rs:

• Main manifest structure definitions
• Public API surface
• Module re-exports and orchestration

Benefits

• Adherence to project's ≤400-line standard
• Improved separation of concerns
• Better code organization and maintainability
• Enhanced testability of individual components

Context

This refactoring requirement was identified during code review of PR #152.

Reference: #152