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_record_e2e.rs

@@ -23,14 +23,12 @@
 //! `e2e.rs::build_and_run_with_rc_stats` (the runtime's fixed
 //! `ailang_rc_stats: allocs=N frees=M live=K` shape).
 
+use ailang_test_support::workspace_root;
 use std::path::PathBuf;
 use std::process::Command;
 
 fn ail_bin() -> &'static str { env!("CARGO_BIN_EXE_ail") }
 fn manifest() -> PathBuf { PathBuf::from(env!("CARGO_MANIFEST_DIR")) }
-fn ws_root() -> PathBuf {
-    manifest().parent().unwrap().parent().unwrap().to_path_buf()
-}
 
 #[derive(Debug)]
 struct RcStats {
@@ -44,7 +42,7 @@ struct RcStats {
 /// `AILANG_RC_STATS=1`, and return the `ailang_ctx_free` readback.
 fn build_link_run_embed(fixture: &str, host_rel: &str, extra_cc: &[&str]) -> RcStats {
     let module = fixture.strip_suffix(".ail").expect("fixture is a .ail file");
-    let fixture_path = ws_root().join("examples").join(fixture);
+    let fixture_path = workspace_root().join("examples").join(fixture);
     let host_c = manifest().join("tests").join(host_rel);
     let outdir = std::env::temp_dir().join(format!(
         "ail-embed-record-e2e-{}-{}",