fix(parser): repair Zig support — grammar nodes were never matched

[geeknik]

Zig was registered in EXTENSION_TO_LANGUAGE and the per-language node-type tables, but the mapped types (fn_proto, fn_decl, container_declaration, call_expression, builtin_call_expr) don't exist in the tree-sitter-zig grammar shipped by tree_sitter_language_pack — that grammar emits PascalCase nodes (FnProto, VarDecl, SuffixExpr, FnCallArguments, FieldOrFnCall, BUILTINIDENTIFIER, ContainerDecl, TestDecl). Result: every Zig file produced only its File node, with zero functions, structs, imports, or calls.

Rather than rename and lose information, route Zig through a constructs handler (mirroring the Lua/Elixir pattern) so we can bridge FnProto's sibling Block, distinguish struct/union/enum/opaque ContainerDecl kinds, and split @import out as IMPORTS_FROM rather than CALLS.

• Empty _CLASS_TYPES/_FUNCTION_TYPES/_IMPORT_TYPES/_CALL_TYPES for "zig" with comments explaining why generic dispatch can't be used.
• Add _extract_zig_constructs dispatch in _extract_from_tree.
• Implement _handle_zig_fn_decl, _handle_zig_var_decl, _handle_zig_test_decl, _zig_extract_import_target, and _extract_zig_calls_in_subtree. The call walker emits CALLS for any SuffixExpr/FieldOrFnCall whose direct children include FnCallArguments, resolving the callee to the immediately preceding IDENTIFIER or BUILTINIDENTIFIER. @import is filtered out so it isn't double-counted.
• Add a Zig branch to _resolve_module_to_file: relative @import("./x.zig") resolves to an absolute file path; std and build-graph module names stay unresolved (resolving the latter would require parsing build.zig).
• Add tests/fixtures/sample_zig.zig + sample_zig_util.zig covering top-level fns, struct with methods, enum, tagged union, std + relative @import, @intcast builtin call, and a labelled test block.
• Add TestZigParsing to tests/test_multilang.py (11 tests).

Verified on a 41-file Zig project: before the fix, build produced 0 nodes / 0 edges; after, 393 unique nodes (214 Function, 124 Test, 55 Class) and 2839 edges (2353 CALLS, 393 CONTAINS, 93 IMPORTS_FROM), with hub detection correctly surfacing Reader.next, sanitize helpers, and Response.deinit as the most-called sites.

[geeknik]

Those CI failures aren't from my PR.