feat(joinir): Phase 112 strict guard for StepTree required_caps

- Add control_tree_capability_guard.rs with check(tree, func_name, strict, dev)
- Allowlist: If, NestedIf, Loop, Return, Break, Continue
- Deny (strict): NestedLoop, TryCatch, Throw, Lambda, While, ForRange, Match, Arrow
- Wire into lower_function_body() (strict-only check)
- Error format: [joinir/control_tree/cap_missing/<Cap>] with 1-line Hint
- Unit tests: nested_loop_rejects, if_only_passes, strict_false_passes
- Default behavior unchanged (strict=false always Ok)

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

src/mir/control_tree/mod.rs

@@ -7,6 +7,7 @@
 mod step_tree;
 
 pub use step_tree::{
-    AstSummary, StepNode, StepStmtKind, StepTree, StepTreeBuilderBox, StepTreeFeatures,
+    AstSummary, ExitKind, StepCapability, StepNode, StepStmtKind, StepTree, StepTreeBuilderBox,
+    StepTreeContract, StepTreeFeatures, StepTreeSignature,
 };