…gs! (#858)

## Summary

Fixes #857

When `find_snapshot_macro(line)` was called for a line inside a macro
like `with_settings!`, the `scan_nested_macros` function would scan all
nested tokens and find all `@"..."` patterns. Since
`try_extract_snapshot` always overwrote the result, the **last**
snapshot found would win regardless of which line we were searching for.

This caused:
1. All searches for lines inside `with_settings!` returned the same
(last) snapshot position
2. The duplicate detection logic discarded all but the first pending
snapshot
3. The first pending snapshot's content got written to the last
snapshot's position

**Example from the issue:**
```rust
insta::with_settings!({filters => vec![...]}, {
    assert_snapshot!(":12345\n\nabc", @"");  // line 10
    assert_snapshot!(":12345\n\nabc", @"");  // line 11
    assert_snapshot!("", @"");               // line 12
    assert_snapshot!("", @"");               // line 13 - incorrectly got content from line 10!
});
```

## The Fix

Instead of filtering during scanning, we now:
1. **Collect all snapshots** with their macro boundaries (start/end
line)
2. **Filter at the end** to find the one whose macro span contains the
target line

This approach is more general because:
- The scanning logic collects everything uniformly
- The line-matching logic is centralized in one place
- It works at any nesting depth

## Test plan

- [x] All existing tests pass (111 tests)
- [x] Added regression test
`test_find_snapshot_macro_multiple_in_with_settings`
- [x] Manually verified with the reproduction case from the issue

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

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

## Summary

- Fix inline snapshot corruption when multiple snapshots appear inside
`with_settings!` macro #858

## Test plan

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

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

> _This was written by Claude Code on behalf of max-sixty_

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