feat: add .xs file support and fix Perl call extraction

[potatogim]

Summary

• Map .xs (Perl XS) extension to the C parser — XS files are predominantly
  C code, so tree-sitter-c captures functions, structs, and #include directives
  correctly while gracefully ignoring XS-specific macros (MODULE, CODE:, etc.)
• Fix two silent bugs in Perl call extraction that caused all method/built-in calls
  to be dropped from the call graph
• Add TestXSParsing test class and strengthen TestPerlParsing assertions

Bug Fixes

$obj->method() calls were silently ignored

_get_call_name() assumed all languages use an identifier child for call
expressions. Perl's method_call_expression uses a method child instead,
so the function returned None for every OOP method call — no CALLS edges
were produced for arrow-operator calls.

Built-in calls (print, bless, die, warn) were not captured

tree-sitter-perl parses Perl built-ins as ambiguous_function_call_expression
nodes, which were missing from _CALL_TYPES["perl"]. These calls were
silently skipped during graph construction.

Changes

File	Change
code_review_graph/parser.py	Add .xs → c to EXTENSION_TO_LANGUAGE
code_review_graph/parser.py	Add ambiguous_function_call_expression to _CALL_TYPES["perl"]
code_review_graph/parser.py	Handle method_call_expression in _get_call_name()
tests/fixtures/sample.xs	New XS fixture (struct + C functions + XS macros)
tests/fixtures/sample.pl	Use $self->speak() to exercise method call path
tests/test_multilang.py	Add TestXSParsing (6 tests); strengthen Perl call assertions

Test Plan

• TestXSParsing — 6 new tests: language detection, struct, functions,
  includes, calls, contains
• TestPerlParsing.test_finds_calls now asserts specific targets
  (speak via method call, bless via ambiguous call)
• Full test suite: 508 passed, 5 skipped — no regressions