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/1210-ffi-02-small-struct.stdout

@@ -0,0 +1,12 @@
+vec2 make   = (1.500000, 2.500000)
+vec2 swap   = (2.500000, 1.500000)
+vec2 sum    = 4.000000
+vec4f make  = (1.000000, 2.000000, 3.000000, 4.000000)
+vec4f rev   = (4.000000, 3.000000, 2.000000, 1.000000)
+vec4f sum   = 10.000000
+pair64 make = (100, 200)
+pair64 swap = (200, 100)
+pair64 sum  = 300
+quad32 make = (10, 20, 30, 40)
+quad32 rev  = (40, 30, 20, 10)
+quad32 sum  = 100