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/lower_to_mir_ty.rs

@@ -8,16 +8,7 @@
 use ailang_check::elaborate_workspace;
 use ailang_mir::{Callee, MTerm, MirWorkspace, Mode, StrRep};
 use ailang_surface::load_workspace;
-use std::path::{Path, PathBuf};
-
-/// `<repo>/examples` — resolved from this crate's manifest dir,
-/// cwd-independent (same pattern as `method_collision_pin.rs`).
-fn examples_dir() -> PathBuf {
-    Path::new(env!("CARGO_MANIFEST_DIR"))
-        .parent().expect("crates/ailang-check has parent crates/")
-        .parent().expect("crates/ has parent repo root")
-        .join("examples")
-}
+use ailang_test_support::examples_dir;
 
 /// Load a witness fixture (prelude injected) and elaborate it to MIR.
 fn elaborate_fixture(module: &str) -> MirWorkspace {