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.lock

@@ -11,6 +11,7 @@ dependencies = [
  "ailang-core",
  "ailang-prose",
  "ailang-surface",
+ "ailang-test-support",
  "anyhow",
  "blake3",
  "clap",
@@ -25,6 +26,7 @@ dependencies = [
  "ailang-core",
  "ailang-mir",
  "ailang-surface",
+ "ailang-test-support",
  "indexmap",
  "serde",
  "serde_json",
@@ -48,6 +50,7 @@ name = "ailang-core"
 version = "0.0.1"
 dependencies = [
  "ailang-surface",
+ "ailang-test-support",
  "blake3",
  "indexmap",
  "serde",
@@ -73,6 +76,7 @@ version = "0.0.1"
 dependencies = [
  "ailang-core",
  "ailang-surface",
+ "ailang-test-support",
 ]
 
 [[package]]
@@ -81,11 +85,16 @@ version = "0.0.1"
 dependencies = [
  "ailang-core",
  "ailang-kernel",
+ "ailang-test-support",
  "serde_json",
  "tempfile",
  "thiserror",
 ]
 
+[[package]]
+name = "ailang-test-support"
+version = "0.0.1"
+
 [[package]]
 name = "anstream"
 version = "1.0.0"