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/e2e.rs

@@ -3,6 +3,7 @@
 //! This test guards the most important property of the entire pipeline:
 //! AST → typecheck → LLVM IR → clang → binary → correct stdout.
 
+use ailang_test_support::workspace_root;
 use std::path::Path;
 use std::process::Command;
 
@@ -2801,15 +2802,6 @@ fn merge_prose_prints_framed_prompt() {
 
 // ---------- ext-cli.1: CLI accepts `.ail` (Form A) source files ----------
 
-fn workspace_root() -> std::path::PathBuf {
-    Path::new(env!("CARGO_MANIFEST_DIR"))
-        .parent()
-        .unwrap()
-        .parent()
-        .unwrap()
-        .to_path_buf()
-}
-
 /// Property: `ail check` accepts a Form-A `.ail` source file (not just
 /// the Form-B `.ail.json` canonical form) and exits zero on a
 /// well-formed program. Guards against the post-rename misleading-JSON