feat(php-shim): give ported exceptions PHP's class hierarchy

Ported exceptions were flat structs reached with `downcast_ref`, so
Composer's `catch (\RuntimeException $e)` only matched the exact leaf
type and `get_class($e)` had nothing to report. Each exception now
embeds an instance of the class it extends and travels inside an
`AnyThrowable`; `Catch::catch`/`catch_mut` walk that chain, and
`PhpClass::php_class_name` yields the PHP FQCN.

Dropping the `std::error::Error` impls from the exception types leaves
`AnyThrowable` as the only route into an `anyhow::Error`, so the walk
cannot be bypassed. A `no_exception_downcast` linter catches the
`downcast::<X>()` calls that would now silently answer `None`.

Three sites change behavior as a result: the `TransportException`
exit-code override reaches `MaxFileSizeExceededException`, the
`catch (\LogicException)` in findSimilar() reaches its subclasses, and
rendered exception titles carry the real class name rather than a
guess. `get_class_err()` is no longer a `todo!()`, which re-enables
FilesystemRepositoryTest::testCorruptedRepositoryFile.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

1 files changed, 9 insertions, 0 deletions

diff --git a/scripts/linters/lint b/scripts/linters/lint
index 44ea7f6d..5f59945d 100755
--- a/scripts/linters/lint
+++ b/scripts/linters/lint
@@ -9,6 +9,7 @@ use Shirabe\Lint\Linters\CargoWorkspaceDependencies;
 use Shirabe\Lint\Linters\ContiguousUseBlock;
 use Shirabe\Lint\Linters\NoBannedUse;
 use Shirabe\Lint\Linters\NoDecorativeSectionComment;
+use Shirabe\Lint\Linters\NoExceptionDowncast;
 use Shirabe\Lint\Linters\NoFormatTrailingComma;
 use Shirabe\Lint\Linters\NoModRs;
 use Shirabe\Lint\Linters\NoStdCollectionsMaps;
@@ -26,6 +27,14 @@ $runner = new Runner($rootDir, [
     [new NoDecorativeSectionComment(), [
         'crates/shirabe-semver/src/version_parser.rs',
     ]],
+    [new NoExceptionDowncast(), [
+        // Defines the box and the walk the rule points at.
+        'crates/shirabe-php-shim/src/exception.rs',
+        // Downcast a panic payload, not an error: PHP's error handler throws an \ErrorException
+        // where these call sites `catch` it.
+        'crates/shirabe/src/cache.rs',
+        'crates/shirabe/src/util/filesystem.rs',
+    ]],
     [new NoFormatTrailingComma(), [
         'crates/shirabe/src/package/loader/root_package_loader.rs',
         'crates/shirabe-spdx-licenses/src/spdx_licenses.rs',