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

examples/ownership_checked.bux

@@ -0,0 +1,35 @@
+import Std::Io::{PrintLine, PrintInt};
+import Std::Test::{Test_AssertEqInt, Test_Pass};
+
+func UncheckedInc(p: *int) {
+    *p = *p + 1;
+}
+
+@[Checked]
+func Inc(p: &mut int) {
+    *p = *p + 1;
+}
+
+@[Checked]
+func Get(p: &int) -> int {
+    return *p;
+}
+
+@[Checked]
+func OkBorrow() -> int {
+    var x: int = 10;
+    Inc(&x);
+    return Get(&x);
+}
+
+func Main() -> int {
+    var n: int = 1;
+    UncheckedInc(&n);
+    Test_AssertEqInt(n, 2);
+    let r: int = OkBorrow();
+    Test_AssertEqInt(r, 11);
+    PrintInt(r);
+    PrintLine("");
+    Test_Pass("ownership_checked");
+    return 0;
+}