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/basic/expected/0025-basic-literals.stdout

@@ -0,0 +1,18 @@
+=== 1. Literals ===
+decimal: 42
+hex: 255
+binary: 10
+float: 3.140000
+f64: 2.718281
+true: true
+false: false
+escapes: hello	world
+multiline: line1
+line2
+heredoc: raw heredoc
+
+undef-then-set: 77
+enum-lit: .green
+null-ptr: null
+string-len: 5
+empty-string: 0