fix(r8-tests): Correctly handle no-line mappings

[romtsn]

• Improved stacktrace parsing to handle frames that omit a line number (e.g. (Unknown), (Unknown Source), (Foo.java)) by treating them as line 0 so they can be retraced.
• Enhanced retracing behavior for unknown-line frames:
  • Prefer base (no-line) mappings when the stacktrace frame has no line info.
  • Still prefer line-specific mappings when a concrete line is present (skipping 0:0 / base entries appropriately).
  • Added best-effort class remapping even when the method name isn’t mapped (replicates symbolicator behaviour -- we can probably ditch it from symbolicator once this is shipped)
• Aligned cache behavior with mapper behavior:
  • Cache serialization no longer drops base/no-line member mappings (instead, we check it during remapping)
  • Cache remapping now mirrors mapper rules for unknown-line frames (including deduplication)

The tests in #71 should pass with this PR