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/modules/0775-modules-qualified-generic-missing-member/a.sx

@@ -0,0 +1,3 @@
+// Namespace `a` authors ONLY `Other` — no `Box`. A qualified `a.Box(..)` head
+// must diagnose the missing member, not resolve to another module's `Box`.
+Other :: struct { z: i64; }

examples/modules/0775-modules-qualified-generic-missing-member/b.sx

@@ -0,0 +1,4 @@
+// An unrelated module's generic `Box($T)` (two fields, size 16). The qualified
+// head `a.Box(..)` must NOT silently resolve to this template via the bare
+// global last-wins map.
+Box :: struct($T: Type) { x: T; y: T; }