Test case from issue #827 to verify that multiline snapshots containing
only newlines (no quotes or backslashes) are output as regular strings
rather than raw strings.

(for transparency, claude code made this)

## Summary

The documentation showed `test_runner_fallback` as the config key, but
the code expected `runner_fallback`. This is consistent with other keys
under `test:`:

```yaml
test:
  runner: nextest          # not test_runner
  runner_fallback: true    # not test_runner_fallback (was documented wrong)
  unreferenced: warn       # not test_unreferenced
  auto_review: true        # not test_auto_review
```

## Changes

- Fix documentation to show correct key: `runner_fallback`
- Add integration test verifying config file setting works
- Fix typo in error message (`INSTA_RUNNER_FALLBACK` →
`INSTA_TEST_RUNNER_FALLBACK`)

## Example

```yaml
# .config/insta.yaml
test:
  runner: nextest
  runner_fallback: true  # this is the correct key
```

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Co-authored-by: Claude <[email protected]>

## Summary

This adds support for the `INSTA_PENDING_DIR` environment variable,
which allows pending snapshots to be written to a separate directory
while keeping the source tree read-only. This is particularly useful for
hermetic build systems like Bazel.

When `INSTA_PENDING_DIR` is set:
- Pending snapshots (`.snap.new` and `.pending-snap` files) are written
to the specified directory, preserving the same relative structure as
the workspace
- `cargo-insta` correctly discovers and maps these pending snapshots
back to their target locations in the source tree
- External test paths (e.g., `path = "../tests/lib.rs"`) are rejected
with a clear error, as they would escape the pending directory

### Key changes

- **insta**: Add `get_pending_dir()` and `pending_snapshot_path()` in
`env.rs`
- **insta**: Update `runtime.rs` to use `pending_snapshot_path` for all
snapshot file operations
- **insta**: Create parent directories in `snapshot.rs` when pending dir
is set
- **cargo-insta**: Update `find_pending_snapshots()` in `walk.rs` to
support path mapping between `pending_root` and `target_root`
- **cargo-insta**: Simplify `load_snapshot_containers()` in `cli.rs` to
use unified loop for both hermetic and default modes
- **cargo-insta**: Add comprehensive tests in `pending_dir.rs` (9 new
tests)

## Test plan

- [x] All 93 cargo-insta tests pass
- [x] New `pending_dir` tests cover: file snapshots, inline snapshots,
accept, reject, auto-creation, update existing, check mode, outside
workspace, and external test path rejection
- [x] Clippy clean
- [x] Pre-commit passes

🤖 Generated with [Claude Code](https://claude.com/claude-code)

---------

Co-authored-by: Claude <[email protected]>

## Summary

- Add `INSTA_PENDING_DIR` environment variable for Bazel and other
hermetic build systems
- Fix documentation for `test.runner_fallback` config key
- Fix missing attribution for #834

## Test plan

- [x] All tests pass
- [x] Pre-commit passes

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-authored-by: Claude <[email protected]>