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/0030-basic-builtins.stdout

@@ -0,0 +1,31 @@
+=== 7. Builtins ===
+out-ok
+sqrt: 3.000000
+sqrt-f64: 4.000000
+sizeof-i32: 4
+sizeof-f64: 8
+sizeof-struct: 8
+alignof-u8: 1
+alignof-i32: 4
+alignof-i64: 8
+alignof-struct: 4
+typeof: int
+typeof-float: float
+typeof-string: string
+typeof-bool: bool
+typeof-struct: struct
+typeof-enum: enum
+typename: Point
+fieldcount: 2
+fieldcount-enum: 3
+fieldname0: x
+fieldname1: y
+fieldname-enum0: red
+fieldname-enum2: blue
+fieldval0: 11
+fieldval1: 22
+fieldidx: 1
+fieldidx-tagged: 0
+fieldidx-tagged2: 2
+cast: 3
+cast-int-f64: 42.000000