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.

Cargo.toml

@@ -8,6 +8,7 @@ members = [
     "crates/ailang-kernel",
     "crates/ailang-surface",
     "crates/ailang-prose",
+    "crates/ailang-test-support",
     "crates/ail",
 ]
 # `ail-embed/` is intentionally NOT a member: it depends on the
@@ -39,6 +40,7 @@ ailang-codegen = { path = "crates/ailang-codegen" }
 ailang-kernel = { path = "crates/ailang-kernel" }
 ailang-surface = { path = "crates/ailang-surface" }
 ailang-prose = { path = "crates/ailang-prose" }
+ailang-test-support = { path = "crates/ailang-test-support" }
 
 [profile.release]
 lto = "thin"