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/diagnostics/expected/1125-diagnostics-reserved-name-method-param.stderr

@@ -0,0 +1,11 @@
+error: 'u8' is a reserved type name and cannot be used as an identifier
+  --> examples/diagnostics/1125-diagnostics-reserved-name-method-param.sx:15:28
+   |
+15 |     greet :: (self: *Self, u8: i64) -> i64 {
+   |                            ^^
+
+error: 'i16' is a reserved type name and cannot be used as an identifier
+  --> examples/diagnostics/1125-diagnostics-reserved-name-method-param.sx:23:27
+   |
+23 |     bump :: (self: *Self, i16: i32) {
+   |                           ^^^