document export bindings and script host visibility

Author: pchalamet

docs/embedding-fscript-language.md

@@ -57,13 +57,14 @@ let result = FScript.evalWithExterns [ toUpperExtern ] typed
 ## Loading once and invoking by name
 
 Use `FScript.Runtime.ScriptHost` when a host needs reusable loading and direct function invocation.
+Only top-level exported bindings are exposed through this API.
 
 ```fsharp
 open FScript.Language
 open FScript.Runtime
 
 let externs = Registry.all { RootDirectory = "." }
-let loaded = ScriptHost.loadSource externs "let add x y = x + y"
+let loaded = ScriptHost.loadSource externs "export let add x y = x + y"
 let result = ScriptHost.invoke loaded "add" [ VInt 1L; VInt 2L ]
 ```
 

docs/syntax-and-indentation.md

@@ -8,6 +8,7 @@ This document describes the concrete syntax accepted by the interpreter and the
 - Top-level statements are:
   - `type` / `type rec` declarations
   - `let` / `let rec` bindings
+  - `export let` / `export let rec` bindings
   - expressions
 - Parsing is layout-aware: the lexer emits `Indent`/`Dedent` tokens from leading whitespace.
 - Comments use `//` line-comment syntax.
@@ -23,6 +24,7 @@ This document describes the concrete syntax accepted by the interpreter and the
   - `let x = expr`
   - `let rec f x = ... and g y = ...`
   - nested via blocks
+  - `export` is only valid on top-level `let` bindings
 - Conditionals:
   - `if cond then a else b`
   - `elif` desugars to nested `if`