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/1115-diagnostics-cast-value-param-rejected.sx

@@ -1,12 +0,0 @@
-// `cast(T)` where `T` is a value parameter declared `: Type` (without the `$`
-// generic sigil) is rejected with the generic-param hint. Previously it
-// silently cast to a fabricated empty struct (an unknown *literal* cast target
-// already errored via value resolution, but the value-param case was silent).
-// Regression (issue 0064, cast position). Expected: tailored error; exit 1.
-conv :: (T: Type, x: i32) -> i32 {
-    return cast(T) x;
-}
-
-main :: () -> i32 {
-    return conv(i32, 5);
-}