Expose debug_meta.images[].code_file basename as a searchable tag (e.g. code_filename)

### Problem Statement

Native crashes are invisible to executable-name searches when the
faulting frame isn't owned by the binary in question.

Today, the only way to find crashes associated with a specific
executable is `stack.package:*foo.exe*`, which only matches when
`foo.exe` owns at least one frame in the crash stack. If `foo.exe`
loads a DLL that faults on its own worker thread, no `foo` frames
appear on the crashing stack — and the event becomes unfindable in
Issues/Discover, even though Sentry already stores `foo.exe` in the
event's `debug_meta.images[].code_file` as a loaded module.

Concrete customer scenario: they ship `iwproxygen.exe`, which loads
a DLL that runs on its own threads and occasionally faults. They
need to surface every crash where `iwproxygen.exe` was loaded into
the process — regardless of which binary owns the faulting frame —
and currently cannot.

Related: getsentry/sentry#83608 (same underlying ask).

### Solution Brainstorm

Expose the **basename** of each loaded image from
`debug_meta.images[].code_file` (just the filename + extension, not
the full path) as a searchable tag/field in Issues and Discover.

Suggested field name: `code_filename`. Usage:

```
code_filename:iwproxygen.exe
```

Basename rather than full path because:

* Low cardinality — full paths vary by machine, install dir, drive
letter, and locale; basenames don't
* Plays nicely with search autocomplete
* Mirrors how `stack.package` already behaves on native in spirit

The data is already stored on every native event — this should be
purely an indexing/exposure change on the platform side, no SDK
work required.

### Product Area

Issues

Start by tracing how debug_meta.images[].code_file is indexed and exposed to search in Issues and Discover. Verify the basename is available as code_filename and that searches such as code_filename:iwproxygen.exe find events where the executable is loaded but owns no faulting frame.

Written by the indexing model from the issue text.