test: group examples into per-category folders

Move examples/*.sx and their expected/ snapshots into per-category
subfolders (examples/<category>/...). Folder = leading filename token,
with ffi-objc/ffi-jni kept whole; filenames are unchanged. The corpus
runner and LSP sweep now discover each category's expected/ dir, while
issues/ stays flat. Example 1058's repo-root-relative companion import
is made file-relative. Path strings embedded in 164 snapshots were
regenerated (path-only changes). Test-layout docs in CLAUDE.md updated.

examples/protocols/0400-protocols-impl-for-builtin.sx

@@ -0,0 +1,30 @@
+// impl Protocol for built-in scalar types (f32, i64, bool, u32, ...) —
+// both static dispatch (`f32.lerp(...)`) and protocol-boxed dispatch via
+// `#inline` erasure.
+
+Lerpable :: protocol #inline {
+    lerp :: (self: *Self, b: Self, t: f32) -> Self;
+}
+
+impl Lerpable for f32 {
+    lerp :: (self: f32, b: f32, t: f32) -> f32 { self + (b - self) * t }
+}
+
+do_lerp :: (a: Lerpable, b: f32, t: f32) -> f32 {
+    a.lerp(b, t)
+}
+
+main :: () -> void {
+    // Static call through impl
+    result := f32.lerp(0.0, 10.0, 0.5);
+    print("lerp(0, 10, 0.5) = {}\n", result);
+
+    // Protocol dispatch through #inline erasure
+    val : f32 = 0.0;
+    p : *f32 = @val;
+    l : Lerpable = xx p;
+    result2 := do_lerp(l, 10.0, 0.25);
+    print("lerp(0, 10, 0.25) = {}\n", result2);
+}
+
+#import "modules/std.sx";