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-surface/tests/round_trip.rs

@@ -32,14 +32,7 @@
 //! `every_ail_fixture_matches_its_json_counterpart` (no longer
 //! expressible: only one form is hand-authored post-iter).
 
-use std::path::PathBuf;
-
-fn examples_dir() -> PathBuf {
-    // `CARGO_MANIFEST_DIR` is the surface crate root; examples live two
-    // levels up.
-    let crate_dir = PathBuf::from(env!("CARGO_MANIFEST_DIR"));
-    crate_dir.parent().unwrap().parent().unwrap().join("examples")
-}
+use ailang_test_support::{examples_dir, list_ail_fixtures};
 
 // Retired iter form-a.1 T9:
 // - `list_json_fixtures` helper (walked `.ail.json` for the two retired tests below).
@@ -51,34 +44,6 @@ fn examples_dir() -> PathBuf {
 //   only one form is hand-authored post-iter; counterparts are derived in-process
 //   via `ail parse`.
 
-/// Fixtures that intentionally do NOT parse — the `#55` cutover
-/// reject corpus. Negative fixtures whose whole point is that the
-/// parser rejects them, so the parse/round-trip properties below are
-/// not meaningful for them and the corpus scan must skip them.
-/// Mirrors the same list in `crates/ail/tests/roundtrip_cli.rs`.
-///
-/// - `bare_slot_reject.ail`: a bare fn-type slot with no mode. The
-///   binary-ParamMode parser rejects it ("fn-type slot requires a
-///   mode: write (own T) or (borrow T)").
-const NON_PARSEABLE_FIXTURES: &[&str] = &["bare_slot_reject.ail"];
-
-fn list_ail_fixtures() -> Vec<PathBuf> {
-    let dir = examples_dir();
-    let mut paths: Vec<PathBuf> = std::fs::read_dir(&dir)
-        .unwrap_or_else(|e| panic!("read_dir({}): {e}", dir.display()))
-        .filter_map(|entry| entry.ok())
-        .map(|e| e.path())
-        .filter(|p| {
-            p.file_name()
-                .and_then(|n| n.to_str())
-                .map(|n| n.ends_with(".ail") && !NON_PARSEABLE_FIXTURES.contains(&n))
-                .unwrap_or(false)
-        })
-        .collect();
-    paths.sort();
-    paths
-}
-
 /// Pins **idempotency-under-print** (property 2 of the post-form-a
 /// Roundtrip Invariant, design/contracts/0009-roundtrip-invariant.md): for every
 /// well-formed `.ail` text `t`, the composition `parse → print → parse`