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/ir_snapshot.rs

@@ -2,6 +2,7 @@
 //! changes to the LLVM IR. Snapshots in tests/snapshots/. Update with
 //! UPDATE_SNAPSHOTS=1 cargo test ir_snapshot_.
 
+use ailang_test_support::examples_dir;
 use std::path::{Path, PathBuf};
 use std::process::Command;
 
@@ -44,15 +45,6 @@ fn snapshots_dir() -> PathBuf {
     Path::new(manifest_dir).join("tests").join("snapshots")
 }
 
-fn examples_dir() -> PathBuf {
-    let manifest_dir = env!("CARGO_MANIFEST_DIR");
-    Path::new(manifest_dir)
-        .parent()
-        .unwrap()
-        .parent()
-        .unwrap()
-        .join("examples")
-}
 
 fn first_diff_line(a: &str, b: &str) -> Option<(usize, String, String)> {
     let mut la = a.lines();