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/1122-diagnostics-reserved-name-impl-method.stderr

@@ -0,0 +1,11 @@
+error: 'u8' is a reserved type name and cannot be used as an identifier
+  --> examples/diagnostics/1122-diagnostics-reserved-name-impl-method.sx:19:24
+   |
+19 |     go :: (self: *Box, u8: i64) {
+   |                        ^^
+
+error: 'i2' is a reserved type name and cannot be used as an identifier
+  --> examples/diagnostics/1122-diagnostics-reserved-name-impl-method.sx:20:9
+   |
+20 |         i2 := Box.{ total = 1 };
+   |         ^^