feat: add 'simplify variable initialization' code action

[ddmoney420]

Summary

Implements a new refactor code action that simplifies verbose variable initialization patterns, as requested in #2098.

Transformations

1. @as builtin to type annotation:

// Before
const foo = @as(T, value);
// After
const foo: T = value;

2. Explicit struct init to anonymous:

// Before
const bar = MyType{};
// After  
const bar: MyType = .{};

Implementation Details

• Added generateSimplifyVariableInitCodeActions() handler in src/features/code_actions.zig
• Added isStructInitVariant() helper for struct init node detection
• Integrated into generateCodeActionsInRange() for cursor-triggered refactors
• Action kind: .refactor (not auto-applied, user-triggered)

Edge Cases Handled

• ✅ Skips if declaration already has type annotation
• ✅ Skips struct init with explicit field assignments (T{ .x = 1 })
• ✅ Handles both const and var declarations
• ✅ Supports complex/nested types and pointer types

Test Coverage

Added 8 comprehensive tests covering:

• Basic @as transformation
• Complex type annotations
• Pointer type annotations
• Variable declarations (var vs const)
• Empty struct initialization
• Nested type paths
• Skip conditions (existing annotation, struct with fields)

All 546 tests pass.

Test Plan

• zig build test passes (546/555, 9 skipped as expected)
• zig fmt clean
• Manual verification of code action in editor

Closes #2098

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[leecannon]

Even if the implementation is good (I haven't checked) the LLM markdown files will not be accepted.