fix(parsers/zig): align function_extractor with the real tree-sitter-zig grammar + lazy import

[gadievron]

Pinned against the actually installed grammar (tree-sitter-zig 1.1.2) by
AST-probing it directly; several AST node-type names had drifted. Three defects
in libs/openant-core/parsers/zig/function_extractor.py:

1. Struct extraction was dead and fields could be emitted as functions. Struct
   handling gated on node types the 1.1.2 grammar never emits (VarDecl /
   container_decl / ContainerDecl); the grammar emits variable_declaration and
   struct_declaration, so total_classes was always 0 and the literal
   container_field->method path was unreachable. Corrected the node-type names;
   methods now flow through the recursive walk which only treats
   function_declaration as a unit, so container_field (struct FIELDS) are never
   emitted. The dedicated _extract_struct_methods helper (home of the original
   container_field bug) is removed as superseded.

2. A struct method was emitted twice (qualified Type.method by the eager scan,
   then bare by an unconditional recursion that never threaded current_struct).
   Now thread the struct name into the recursion so a method produces the same
   func_id and the dict de-duplicates instead of producing a bare twin. Also fix
   a name-capture bug in _extract_function (it grabbed the last identifier,
   recording fn init(...) Point { under the return type Point); it now
   captures the first identifier and stops. Regression test pins that the de-dup
   does not drop nested struct / method / @import.

3. tree_sitter_zig was imported at module top level and called at class-body
   time, raising ImportError in any clean install lacking the package, which was
   declared in neither pyproject.toml nor requirements.txt. Load the grammar
   lazily on first FunctionExtractor construction with a clear error message, and
   declare tree-sitter-zig>=1.1.2 in pyproject.toml + requirements.txt.
   (call_graph_builder.py carries the same top-level import but is out of this
   change's scope; the dependency declarations cover it too.)

New tests/parsers/zig/test_zig_extractor_followup.py (loaded via importlib unique
name because function_extractor.py recurs across parser packages). RED at base
6 failed / 1 passed; GREEN 7 passed. Full suite 183 passed, 63 skipped, 0 failed.

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