fix(parsers/zig): canonical call-graph API + correct call extraction and resolution

[gadievron]

Six defects in parsers/zig/call_graph_builder.py:

1. API parity: the zig builder exposed only build()->Dict + save_results(), diverging from the canonical
   CallGraphBuilder (c/php/python/ruby) which has build_call_graph()->None, export()->Dict,
   get_statistics()->Dict, get_dependencies()/get_callers(). Added those methods; build() is retained as
   a back-compat wrapper (build_call_graph()+export()) since the pipeline (zig/test_pipeline.py) consumes
   its return value.

2. Statistics: get_statistics() now also reports avg_in_degree and max_in_degree (previously only the
   out-degree side was emitted).

3. @call extraction: @call(.modifier, fn, args) parses as a builtin_function node, so the wrapped
   function fn was never recorded. It is now extracted as the call target (the @call builtin itself
   stays filtered).

4. Method-call recall + safe resolution: at base a method call obj.method() parses as call_expression
   over a field_expression, but the extractor only handled the non-existent field_access node type,
   so method calls produced no edges. field_expression callees are now extracted. And _resolve_call no
   longer returns ALL same-named candidates on a bare-name collision (which would link a.method() to
   every struct's method); when genuinely ambiguous it returns nothing. Trade-off: lower recall for
   ambiguous bare-name calls, higher precision (no namespace leak).

5. Import-file matching: step 2 matched imp in candidate_file -- an unanchored substring -- so
   'util.zig' matched 'myutil.zig_x/...'. Now matches the import file name exactly (== or path-suffix).

6. Stdlib import filter: @import("std")/("builtin")/("root") are not file imports; they are now
   skipped in resolution (previously they substring-matched candidate paths).

Scope: zig-specific (the c/php/python/ruby builders are separate implementations and are the parity
baseline, not siblings to change). function_extractor.py is unchanged.

Tests: tests/test_zig_call_graph_builder.py (new; the package had no tests, and the tree-sitter-zig
grammar must be installed to run them). Four tests: API parity + in-degree stats, @call extraction,
method-call extraction via field_expression, and exact-import / stdlib-filter / conservative resolution.
RED 4 failed (pre-fix) -> GREEN 4 passed; ruff clean; full suite 180 passed / 63 skipped.

Co-Authored-By: Claude Opus 4.7 (1M context) noreply@anthropic.com