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/embed_rc_global_stats_race.rs

@@ -28,12 +28,8 @@
 //! build of any kernel emits `libailang_rt.a`; the C host links the
 //! runtime archive directly and never calls a kernel.
 
-use std::path::PathBuf;
 use std::process::Command;
 
-fn ws_root() -> PathBuf {
-    PathBuf::from(env!("CARGO_MANIFEST_DIR")).join("../..").canonicalize().unwrap()
-}
 fn ail_bin() -> &'static str { env!("CARGO_BIN_EXE_ail") }
 
 /// 8 threads * 2_000_000 cycles — must match the `#define`s in
@@ -47,7 +43,7 @@ fn global_rc_stats_counters_are_exact_under_thread_contention() {
     let outdir = std::env::temp_dir().join(format!(
         "ail-rc-global-stats-race-{}", std::process::id()));
     std::fs::create_dir_all(&outdir).unwrap();
-    let root = ws_root();
+    let root = ailang_test_support::canonical_workspace_root();
 
     // Any kernel staticlib build emits libailang_rt.a alongside it.
     let st = Command::new(ail_bin())