Add Zig parser support (.zig)

[josephismikhail]

Problem

Ix does not currently parse .zig files. Zig source files are treated as unrecognized and excluded from analysis, meaning systems-level logic, memory management patterns, and build configurations written in Zig are invisible to the system.

Proposed solution

Add a Zig parser that can identify and extract:

• Function definitions (e.g. fn main() void {})
• Struct, union, and enum definitions
• Variable and constant declarations (var, const)
• Import statements (e.g. @import("std"))
• Comptime expressions and comptime blocks
• Error handling patterns (e.g. try, catch, error unions)
• Build system definitions (build.zig)

Support the following file patterns: *.zig

Why it matters

Zig is an emerging systems programming language gaining traction as a modern alternative to C, particularly for performance-critical and low-level code. It also serves as a cross-compilation toolchain used beyond Zig projects themselves. Without parsing Zig, Ix cannot reason about systems-level code or build logic in codebases that have adopted it.

Merge requirements

Before this can be merged to main, the following must pass:

• Tested on 3 repos
• No regressions on existing parsers (ix map on a known TS/JS repo, verify counts unchanged)
• Unit + smoke tests pass (npm test in ix-cli)
• Function definitions (fn) ingested as entities
• Struct, union, and enum definitions ingested
• var and const declarations ingested
• @import statements resolved
• comptime blocks ingested as entities
• CONTAINS chains correctly: file → struct/enum → field/method
• build.zig ingested and build-level declarations visible
• ix text returns results with language: zig
• ix contains returns members for a known Zig struct or enum