Add architecture overview with Mermaid diagram to README

[Copilot]

The README lacked architectural context, forcing contributors to reverse-engineer component relationships from code. This adds a comprehensive Architecture section with structured documentation.

Changes

• Mermaid diagram showing layered architecture:

  • Application Layer → Database API → Core Logic → Storage → Persistence
  • Component relationships with clear data flow paths
  • Color-coded by functional layer

• Component descriptions for each layer with source file references:

  • Database API: db.rs, V2 API
  • Core Logic: Revision Manager, Merkle Trie, Proof Generation, Root Store
  • Storage: Node Store, Linear Storage, Hashers
  • FFI: C bindings and Go wrappers

• Data flow documentation for critical paths:

  • Write path: Proposal creation → merkle hashing → node allocation → disk persistence → root store update
  • Read path: View lookup → trie traversal → cache/disk retrieval
  • Proof generation: Trie traversal → node/hash collection → proof serialization

• Operation flows as ASCII diagrams showing component interaction chains

The existing SVG diagram reference is retained as supplementary material.

Original prompt

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This section details on the original issue you should resolve

<issue_title>Add architecture diagram and overview to README</issue_title>
<issue_description>### Problem
The main README provides terminology and build instructions but lacks a high-level architectural overview. Contributors and integrators find it difficult to understand the overall design and how modules fit together.

Proposed Solution

Create a dedicated "Architecture" section in the top-level README.md with a mermaid diagram illustrating Firewood's major components and their interactions.

Content to Include

• Mermaid diagram showing the relationship between major components:
  • Storage layer
  • Merkle trie implementation
  • Database API
  • FFI layer
  • Proof generation
  • Root store and revision management
• Brief descriptions of each major component
• Data flow explanations
• How components interact during common operations (read, write, commit)

Current State

• No architectural documentation exists
• Component relationships must be inferred from code
• New contributors have difficulty understanding the big picture

References

• firewood/src/ - Main database implementation
• storage/src/ - Storage layer
• ffi/src/ - Foreign function interface
• Existing terminology section in README.md</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Add architecture diagram and overview to README #1399

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