Add documentation infrastructure for RTL symbols

Introduces `RtlDocEntry` and `RtlRegistration` structs to store and
manage documentation for native functions and constants. The
`Environment` struct is updated to include a registry for these
documentation entries.

The `register_native_fn` and `register_constant` methods now return an
`RtlRegistration` builder, allowing for optional chaining of `.doc()`,
`.description()`, and `.examples()` methods before the registration is
finalized when the builder is dropped.

Macros `rtl_fn!` and `rtl_const!` are introduced to simplify the
registration process with documentation.

The MCP server is extended with `get_rtl_docs` and `get_symbol_doc`
tools to expose this documentation externally.

Specific RTL functions and constants (e.g., `+`, `-`, `*`, `/`, `NaN`,
`true`, `false`, `date`, `now`, math functions, `series`, `push`, `len`,
`create-random-ohlc`, `create-ticker`, `pipe`) have been updated to
include their respective documentation using the new infrastructure.

The `Signature` and `StaticType` types have gained `to_doc_string`
methods for generating human-readable documentation strings.

src/ast/mcp_server.rs

@@ -17,6 +17,13 @@ pub struct CodeInput {
     pub code: String,
 }
 
+/// Input struct for tools that look up a single RTL symbol by name.
+#[derive(Debug, serde::Deserialize, schemars::JsonSchema)]
+pub struct SymbolInput {
+    /// The RTL symbol name to look up (e.g. "+", "push", "series").
+    pub name: String,
+}
+
 /// MCP server exposing the Myc compiler as a set of tools for LLMs.
 #[derive(Clone)]
 pub struct MycMcpServer {
@@ -115,6 +122,32 @@ impl MycMcpServer {
     fn get_language_spec(&self) -> String {
         LANGUAGE_SPEC.to_string()
     }
+
+    /// Returns documentation for all documented RTL symbols.
+    #[tool(description = "Return documentation for ALL built-in RTL functions and constants at once. \
+        Prefer get_symbol_doc for individual lookups to save tokens. \
+        Use this only when you need a full overview of the entire standard library.")]
+    fn get_rtl_docs(&self) -> String {
+        let env = Self::make_env();
+        let docs = env.list_rtl_docs();
+        if docs.is_empty() {
+            "No documented RTL symbols found.".to_string()
+        } else {
+            docs.join("\n\n")
+        }
+    }
+
+    /// Returns documentation for a single RTL symbol by name.
+    #[tool(description = "Return the type signature, description, and examples for a single \
+        built-in RTL symbol. Use list_builtins to see all available names, then call this \
+        for each symbol you need details on. More token-efficient than get_rtl_docs.")]
+    fn get_symbol_doc(&self, Parameters(SymbolInput { name }): Parameters<SymbolInput>) -> String {
+        let env = Self::make_env();
+        match env.get_rtl_doc(&name) {
+            Some(doc) => doc,
+            None => format!("No documentation found for symbol '{}'.", name),
+        }
+    }
 }
 
 #[tool_handler]