fix(view): preserve native render failure diagnostics

[y4ho0]

Summary

• preserve native PowerPoint and Word rendering exceptions instead of replacing every failure with an “Office is not installed” message
• distinguish COM class-registration/application availability failures from document-open or rendering failures
• retain the original failure stage and HRESULT in explicit --render native diagnostics
• keep --render auto behavior unchanged: native failures still fall back silently to HTML

Why

When Office is installed and COM activation succeeds, a document-specific open failure such as Invoke(Open) hr=0x80070570 was swallowed by the native backend. The caller then reported that PowerPoint or Word was not installed, which sent users toward the wrong fix and hid the useful HRESULT.

The native backends now surface worker-thread failures while preserving their original stack. Explicit native mode maps those failures to either native_unavailable or native_render_failed; auto mode continues to catch them and use the existing HTML fallback.

Validation

• dotnet build src/officecli/officecli.csproj -c Release — succeeded with 0 errors
• targeted diagnostic harness covered:
  • non-Windows native request → native_unavailable
  • COM class-not-registered → native_unavailable with HRESULT
  • Invoke(Open) hr=0x80070570 → native_render_failed with the original stage and HRESULT
  • native backend returning no image → native_render_failed
• verified that every production caller catches the surfaced backend exception, so --render auto retains its fallback behavior

Closes #326