Handle native frames without source locations

[OskarEichler]

Summary:

• resolve a frame’s source position from the nearest source-bearing frame
• represent an entirely native stack as with line 0
• avoid expanding a missing pathname

Why:
Ruby 4 can expose native frames with nil frame_file. Current formatting passes nil to path handling and fails while rendering the backtrace, as reported in #981. Searching forward through the stack matches the caller location Ruby presents for native frames while retaining a safe fallback.

Verification:

• focused bt command broken by public_send in stacktrace #981 native-frame model passes
• current composite suite remains at upstream baseline: 522 runs, 770 assertions, 13 known Ruby 4 backtrace-layout failures, 0 errors, 2 skips
• gem build and syntax checks pass

Compatibility:
Source-bearing frames are unchanged. Native frames now render instead of raising.