Extend test evaluator to support enums, match, arrays, and choices

[dragonlightintl]

Summary

The zero test direct-frontend evaluator currently only handles expressions composed of integer and bool literals, if statements, and same-file function calls with primitive-only bodies. This PR extends the evaluator to handle the constructs needed for real-world test coverage:

• Qualified enum access: Stage.testing resolves to the case index integer
• Match statements: dispatches on enum discriminant, including _ fallback
• Choice constructors: both nullary (GateOutcome.passed) and with payload (HandoffValidation.invalid("reason"))
• Array literals and indexing: [Stage.testing, Stage.coding] and arr[0]
• Variable assignment and while loops
• Null literals and type casts

Motivation

Without these constructs, only functions with primitive-only bodies (no match, no enums, no shapes) can be tested through zero test. In practice, this means most real pipeline logic — which uses enums and match for exhaustive dispatch — can only be verified via zero check (type-correctness) but not execution-tested.

This change enables tests like:

test "retry_budget_for returns correct budgets" {
    let config = default_config()
    expect(retry_budget_for(config, Stage.testing) == 2)
}

Approach

Extends the existing AST interpreter rather than introducing a compiled-test backend. The evaluator already handled if, let, return, expect, binary operators, shape literals, shape field access, and function calls. This PR adds handlers for the remaining common AST node types.

All changes are in native/zero-c/src/main.c (+208 lines).

What's NOT included

• Choice match dispatch (matching on choice type discriminants) — the STMT_MATCH handler currently only resolves enum types. Choice match would require additional work to resolve choice discriminants.
• Cross-module function resolution in test scope — functions from imported modules still fail. This is a separate issue from expression evaluation.

Test plan

• All existing conformance test fixtures produce identical results (test-blocks.0, test-expected-fail.0, test-expect-runtime-fail.0, test-unexpected-pass.0, test-expect-non-bool.0)
• Builds cleanly with -Wall -Wextra -Wpedantic (no new warnings)
• New enum access, match dispatch, shape construction, array literal, and choice constructor expressions evaluate correctly in test blocks
• Existing zero check behavior unchanged for all example files

[vercel]

@dragonlightintl is attempting to deploy a commit to the Vercel Labs Team on Vercel.

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[dragonlightintl]

Recreating with clean history — stale merge commit in this PR's history.