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_e2e.rs

@@ -4,18 +4,16 @@
 //! + `libailang_rt.a`, run it, assert the `s == 25` assertion holds
 //! (exit 0). This is the whole-milestone existence proof.
 
+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()
-}
 
 #[test]
 fn c_host_calls_exported_scalar_kernel() {
-    let fixture = ws_root().join("examples").join("embed_backtest_step.ail");
+    let fixture = workspace_root().join("examples").join("embed_backtest_step.ail");
     let host_c = manifest().join("tests").join("embed").join("host.c");
     let outdir = std::env::temp_dir()
         .join(format!("ail-embed-e2e-{}", std::process::id()));