feat: Add ROS Action (.action) schema parser

[zhexuany]

Issue Description

Robocodec does not support parsing ROS action definitions (.action files). Actions are used for long-running goal-oriented tasks in ROS (e.g., navigation, manipulation).

Background

A ROS action file defines three messages separated by "---":

# Example: Fibonacci.action
int32 order  # Goal
---
int32[] sequence  # Result
---
float32 progress  # Feedback

The three parts are: Goal, Result, and Feedback.

Proposed Solution

Extend the schema parser to handle .action files with a new ActionSchema type.

Requirements

• Add action_parser.rs module in src/schema/parser/
• Parse action definition format (Goal --- Result --- Feedback)
• Support ROS1 actionlib and ROS2 action formats
• Integrate with unified parser (auto-detect .action files)
• Add ActionSchema type to src/schema/ast.rs
• Support action type references in message schemas
• Add unit tests for various action formats
• Add integration tests with real .action files
• Update documentation

Acceptance Criteria

1. Can parse standard ROS action definitions
2. Correctly separates Goal, Result, and Feedback messages
3. Handles nested types and arrays in action definitions
4. Round-trip parsing preserves all information

Test Cases

• Simple actions (primitive types only)
• Actions with arrays
• Actions with nested message types
• Real-world actions (e.g., control_msgs/FollowJointTrajectory, fibonacci/Fibonacci)

References

• Existing msg parser: src/schema/parser/msg_parser/
• ROS Actions documentation: https://wiki.ros.org/actionlib
• ROS2 Actions: https://docs.ros.org/en/humble/Tutorials/Intermediate/ROS2-Actions/Understanding-ROS2-Actions.html

---

Priority: P1 - Core ROS completeness
Estimated effort: Medium