docs: add technical flow diagrams for audit (#56)

* docs: add technical flow diagrams for audit

Add comprehensive technical sequence diagrams documenting all Trails
contract interaction flows. Created in response to the October 15, 2025
audit call request for detailed contract interaction documentation.

Includes 6 detailed flows with Mermaid diagrams:
1. Cross-chain flow without destination calldata
2. Cross-chain flow with destination calldata (balance injection)
3. Same-chain flow without destination calldata
4. Same-chain flow with destination calldata (direct replacement)
5. Origin chain failure and refund flow
6. Destination chain failure and refund flow

Each flow includes:
- Call batch sequence numbering (Call #1, #2, #3, etc.)
- Key characteristics and use cases
- Detailed Mermaid sequence diagrams
- Implementation notes and semantics
- Contract addresses, function signatures, and call types

Addresses audit requests from Tim Sigl and Andy Lin regarding:
- Contract interaction details with function names
- When injectAndCall() is used vs direct replacement
- TrailsRouterShim purpose and call forwarding
- Sentinel validation and fallback mechanisms

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

* docs: refine technical flow diagrams for clarity

Updated the technical flow diagrams documenting Trails contract interactions by removing specific references to the October 15, 2025 audit call and enhancing the clarity of various sections. Adjusted wording for better readability and streamlined the presentation of key characteristics and implementation notes across different scenarios.

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Co-authored-by: Claude <noreply@anthropic.com>

Author: shunkakinoki