Language: for i in start..end loops

Adds a \`for NAME in START..END { BODY }\` half-open range loop:

    for i in 0..8 {
        total += arr[i]
    }

- Lexer: \`for\`, \`in\` keywords and the \`..\` range operator
- AST: new \`Statement::For\` variant with var/start/end/body
- Parser: \`parse_for\` after \`while\` / \`loop\`
- Analyzer: registers the loop variable as a u8 symbol for the body
  (restoring any shadowed outer symbol afterwards), allocates it via
  the normal RAM allocator, and tracks it as "used"
- IR lowering: desugars to \`var = start; while var < end { body;
  var = var + 1 }\` using a \`for_step\` continue-edge block so
  \`continue\` properly increments the index
- AST codegen: no-op (legacy path doesn't need for loops)
- Tests: parse + full-pipeline integration

https://claude.ai/code/session_01W6eQFStA66EuMKHUFo2rx3

docs/language-guide.md

@@ -484,6 +484,24 @@ while i < 10 {
 }
 ```
 
+### For Loop
+
+The `for` loop iterates over a half-open integer range `[start, end)`:
+
+```
+for i in 0..8 {
+    total += arr[i]
+}
+```
+
+The loop variable is a `u8` scoped to the loop body. Both bounds can
+be any expression that evaluates to `u8` at runtime, including
+constants or variables. The range is half-open, so `0..8` iterates
+`0, 1, 2, ..., 7` (8 iterations). For a closed range, use `0..9`.
+
+The loop is desugared into a `while` loop with an index variable, so
+`break` and `continue` work the same as in any loop body.
+
 ### Loop (Infinite)
 
 ```