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/platform/expected/1643-platform-asm-echo-name.stderr

@@ -0,0 +1,5 @@
+error: redundant asm operand name `eax` — it already names the pinned register; drop the `[eax]`
+  --> examples/platform/1643-platform-asm-echo-name.sx:5:25
+   |
+ 5 | f :: () -> u32 { return asm volatile { "cpuid", [eax] "={eax}" -> u32, "{eax}" = 1 }; }
+   |                         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^