Fix inspect.getsourcelines() indexing bug in --spew trace hook

[veeceey]

Summary

Fixes #3344 -- crash when using --spew with code that has no __file__ in f_globals (e.g. dynamically generated code from the attrs library).

Root cause: inspect.getsourcelines() returns (lines_list, start_lineno) -- a tuple of length 2. The old code did src[lineno], which indexed the tuple itself by the frame's line number instead of indexing the source lines list. This caused:

• AttributeError: 'int' object has no attribute 'rstrip' when lineno == 1 (because src[1] is the integer start_lineno)
• IndexError: tuple index out of range for lineno >= 2

Fix: Properly unpack the tuple and compute the correct offset into the lines list:

# Before (buggy):
src = inspect.getsourcelines(frame)
line = src[lineno]

# After (fixed):
src_lines, src_start = inspect.getsourcelines(frame)
line = src_lines[lineno - src_start]

Also catches IndexError alongside OSError to gracefully fall back when the computed offset is out of bounds.

• Adds a regression test that reproduces the exact crash scenario (dynamically generated code with linecache entries but no __file__ in globals, matching the attrs pattern from the bug report).

Test plan

• New test tests/test_debug.py::TestSpew::test_getsourcelines_indexing passes
• Verified the old code crashes with IndexError: tuple index out of range on the same test scenario
• Verified the fix correctly retrieves source lines for dynamically generated code

🤖 Generated with Claude Code

[veeceey]

Friendly ping - any chance someone could take a look at this when they get a chance? Happy to make any changes if needed.