fix(r8-tests): Align retrace no‑position handling with R8 semantics

[romtsn]

As seen in some of our customers' mappings, frames can come without a filename, so we need handle those cases as well:

 at com.mycompany.ui.DynamicFlowScreenKt.DynamicFlowScreen$lambda$1(:73)
...
 at androidx.compose.runtime.CompositionLocalKt.CompositionLocalProvider(:398)

[loewenheim]

Some of the fields/parameters could use documentation about what they are intended to do.

[codecov]

Codecov Report

❌ Patch coverage is 28.57143% with 50 lines in your changes missing coverage. Please review.
⚠️ Please upload report for BASE (rz/feat/r8-tests-method-overloading@94d628e). Learn more about missing BASE report.

Files with missing lines	Patch %	Lines
src/cache/mod.rs	23.25%	33 Missing ⚠️
src/mapper.rs	37.03%	17 Missing ⚠️

Additional details and impacted files

@@                          Coverage Diff                           @@
##             rz/feat/r8-tests-method-overloading      #81   +/-   ##
======================================================================
  Coverage                                       ?   84.29%           
======================================================================
  Files                                          ?        9           
  Lines                                          ?     3763           
  Branches                                       ?        0           
======================================================================
  Hits                                           ?     3172           
  Misses                                         ?      591           
  Partials                                       ?        0           

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[romtsn]

closing this one for now, as we've adjusted test expectations, see 4c17e66