feat: match guards, HOF inference, ownership C.2/C.3, LSP sema hover

Sessions 18–23 quality work:

- B.3c match arm guards + sequential found-flag lower (bootstrap + selfhost)
- Generic HOF type inference (Array/Iter map/filter/fold without type args)
- Pattern binding shadowing via unique C locals (__pN_src)
- Ownership C.2 exclusive &mut data-flow + C.4 goldens; *p= store-through fix
- Ownership C.3 auto-drop on early return/branches: scoped defers, move-on-return,
  Drop monomorphization, materialize return before Drop
- LSP 0.3.0: hover from real sema types
- Examples and QUALITY_PLAN session log; selfhost-loop identical

tests/error_golden/exclusive_mut_let/bux.toml

@@ -0,0 +1,6 @@
+[Package]
+Name = "exclusive_mut_let"
+Version = "0.1.0"
+Type = "bin"
+[Build]
+Output = "Bin"

tests/error_golden/exclusive_mut_let/expected.err

@@ -0,0 +1,7 @@
+error: type errors in project
+error: cannot mutably borrow 'x': already mutably borrowed
+  --> FILE:5:23
+    |
+ 5 |     let b: &mut int = &x;
+    |                        ^
+   = help: only one active '&mut' borrow is allowed at a time; end the borrow before reuse

tests/error_golden/exclusive_mut_let/src/Main.bux

@@ -0,0 +1,7 @@
+@[Checked]
+func Main() -> int {
+    var x: int = 1;
+    let a: &mut int = &x;
+    let b: &mut int = &x;
+    return 0;
+}