feat: implement built-in State, Reader, and Fail effects

Add runtime support for the State, Reader, and Fail effects that
were already defined in the type system. These effects can now be
used in effectful code blocks.

Changes:
- Add builtin_state and builtin_reader fields to Interpreter
- Implement State.get and State.put in handle_builtin_effect
- Implement Reader.ask in handle_builtin_effect
- Add Reader effect definition to types.rs
- Add Reader to built-in effects list in typechecker
- Add set_state/get_state/set_reader/get_reader methods
- Add 6 new tests for built-in effects
- Add examples/builtin_effects.lux demonstrating usage

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

examples/builtin_effects.lux

@@ -0,0 +1,45 @@
+// Demonstrating built-in effects in Lux
+//
+// Lux provides several built-in effects:
+// - Console: print and read from terminal
+// - Fail: early termination with error
+// - State: get/put mutable state (requires runtime initialization)
+// - Reader: read-only environment access (requires runtime initialization)
+//
+// This example demonstrates Console and Fail effects.
+//
+// Expected output:
+// Starting computation...
+// Step 1: validating input
+// Step 2: processing
+// Result: 42
+// Done!
+
+// A function that can fail
+fn safeDivide(a: Int, b: Int): Int with {Fail} =
+    if b == 0 then Fail.fail("Division by zero")
+    else a / b
+
+// A function that validates input
+fn validatePositive(n: Int): Int with {Fail} =
+    if n < 0 then Fail.fail("Negative number not allowed")
+    else n
+
+// A computation that uses multiple effects
+fn compute(input: Int): Int with {Console, Fail} = {
+    Console.print("Starting computation...")
+    Console.print("Step 1: validating input")
+    let validated = validatePositive(input)
+    Console.print("Step 2: processing")
+    let result = safeDivide(validated * 2, 1)
+    Console.print("Result: " + toString(result))
+    result
+}
+
+// Main function
+fn main(): Unit with {Console} = {
+    let result = run compute(21) with {}
+    Console.print("Done!")
+}
+
+let output = run main() with {}