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/ail/tests/mono_xmod_ctor_pattern.rs

@@ -28,14 +28,7 @@
 //! `workspace_has_typeclasses` gate so every workspace exercises the
 //! mono pass.
 
-use std::path::PathBuf;
-
-fn examples_dir() -> PathBuf {
-    let mut d = PathBuf::from(env!("CARGO_MANIFEST_DIR"));
-    d.pop();
-    d.pop();
-    d.join("examples")
-}
+use ailang_test_support::examples_dir;
 
 /// Property: `monomorphise_workspace` must succeed on a workspace that
 /// (a) flips the typeclass gate (any `Def::Class` + `Def::Instance` is