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/borrow_test.nim

@@ -195,6 +195,46 @@ func Main() -> int {
 """)
     check(not res.hasErrors)
 
+  test "@[Checked] rejects two let-bound &mut of same var":
+    let res = checkSource("""
+@[Checked]
+func Main() -> int {
+  var x: int = 1;
+  let a: &mut int = &x;
+  let b: &mut int = &x;
+  return 0;
+}
+""")
+    check(res.hasErrors)
+    check(res.diagnostics[0].message.contains("already mutably borrowed"))
+
+  test "@[Checked] rejects assign while mutably borrowed":
+    let res = checkSource("""
+@[Checked]
+func Main() -> int {
+  var x: int = 1;
+  let a: &mut int = &x;
+  x = 2;
+  return *a;
+}
+""")
+    check(res.hasErrors)
+    check(res.diagnostics[0].message.contains("mutably borrowed"))
+
+  test "@[Checked] rejects shared borrow while mutably borrowed":
+    let res = checkSource("""
+@[Checked]
+func Main() -> int {
+  var x: int = 1;
+  let a: &mut int = &x;
+  let b: &int = &x;
+  return 0;
+}
+""")
+    check(res.hasErrors)
+    check(res.diagnostics[0].message.contains("shared-borrow") or
+          res.diagnostics[0].message.contains("mutably borrowed"))
+
   test "borrow & expr with shared ref":
     let res = checkSource("""
 struct Point {