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/generics/expected/0201-generics-generic-struct.stdout

@@ -0,0 +1,14 @@
+v1: Vec3{data: [1.000000, 3.000000, 2.000000]}
+v2: [1.000000, 3.000000, 2.000000]
+buff: [0.000000, 2.000000, 3.500000, 4.000000, 0.000000]
+comp: Foo{value: 42, count: 1}
+add: [4.000000, 5.000000, 3.000000]
+v2.x: 1.000000
+v2[1]: 3.000000
+scaled: [2.000000, 6.000000, 4.000000]
+neg: [-1.000000, -3.000000, -2.000000]
+sqrt(9): 3.000000
+.user(0.500000)
+4
+16
+8