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/1163-diagnostics-array-const-index-oob.stderr

@@ -0,0 +1,11 @@
+error: index 9 is out of bounds for constant 'K' (4 elements)
+  --> examples/diagnostics/1163-diagnostics-array-const-index-oob.sx:10:10
+   |
+10 |     b : [K[9]]u8 = ---;
+   |          ^^^^
+
+error: array dimension must be a compile-time integer constant
+  --> examples/diagnostics/1163-diagnostics-array-const-index-oob.sx:10:10
+   |
+10 |     b : [K[9]]u8 = ---;
+   |          ^^^^