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/ffi/expected/1213-ffi-05-string-args.stdout

@@ -0,0 +1,7 @@
+strlen("hello")     = 5
+first_byte           = 104
+strlen(sx string)    = 14
+sum_bytes([10,20,30,40]) = 100
+bytes[1] after write = 99
+static greeting len  = 12
+static greeting [0]  = 104