refactor(test): hoist duplicated test helpers into ailang-test-support

closes #60

The fixture-corpus filter (NON_PARSEABLE_FIXTURES + list_ail_fixtures)
was copy-pasted across three test crates and drifted out of sync as
new reject fixtures landed; the examples_dir / workspace_root path
walk was reimplemented ~30 more times across the suite, under two
spellings (workspace_root / ws_root).

Introduce crates/ailang-test-support — a zero-dependency, dev-only
leaf crate — as the single home for these helpers:
- NON_PARSEABLE_FIXTURES, list_ail_fixtures
- workspace_root, examples_dir
- canonical_workspace_root (symlink-resolved variant, for the tsan/
  race tests that feed the root into native build steps)

All integration-test copies across ailang-core, ailang-surface,
ailang-prose, ailang-check, and ail now import from the shared crate;
the local definitions and their now-orphaned path imports are removed.
Path helpers resolve via the support crate's own CARGO_MANIFEST_DIR,
so they return the correct absolute paths from any caller.

ail_bin helpers are intentionally left in place: they depend on
env!("CARGO_BIN_EXE_ail"), which Cargo only defines when compiling the
ail crate's own integration tests, so they cannot move to a shared
crate.

Behaviour-identical: same paths, same fixture lists; full workspace
test suite green. Net -177 LOC.

crates/ailang-check/tests/workspace.rs

@@ -6,7 +6,7 @@
 
 use ailang_check::{check_workspace, Severity};
 use ailang_surface::load_workspace;
-use std::path::Path;
+use ailang_test_support::examples_dir;
 
 /// assert that the linearity check's `suggested_rewrites`
 /// payload is non-empty AND each `replacement` parses as a form-A AILang
@@ -30,10 +30,6 @@ fn assert_suggested_rewrites_well_formed(d: &ailang_check::Diagnostic) {
     }
 }
 
-fn examples_dir() -> std::path::PathBuf {
-    let manifest = env!("CARGO_MANIFEST_DIR");
-    Path::new(manifest).parent().unwrap().parent().unwrap().join("examples")
-}
 
 #[test]
 fn happy_path_resolves_qualified_import() {