`src/tool/registry.rs` — `ToolRegistry` and `FnTool`

The name→tool phone book, plus the function-adapter for quick tools.

Key items

• ToolRegistry — register (same name overwrites with a warn, original position kept), get, contains, tool_names (sorted), all_schemas (order unspecified), all_tools (registration order, includes unsafe), concurrent_safe_tools, len/is_empty. Send + Sync — share as Arc.
• FnTool — public fields + builder: new(name, description, schema, tool_fn), .concurrency_safe(), .with_concurrency_check(fn), .with_resource_key(fn), .read_only(), .with_system_prompt(...).

Behavior notes

• The engine advertises all_schemas() to the model and classifies replies against tool_names(); unknown names are pre-answered “not available” without dispatch.
• No iteration trait — use all_tools().
• FnTool’s per-input concurrency check overrides the static flag when present.

Deep dive: Tools.