feat: add structured edit previews to fix-plan JSON

[gke0op]

Summary

zero fix --plan --json now includes an edits array on each fix object, showing the exact line, old text, and new text that would change.

This closes the gap between --plan (no edit info) and --patch/--apply (which include patches). Agents can now preview the exact diff before deciding to apply.

Example output

Before:

{
  "fixes": [{
    "id": "make-binding-mutable",
    "appliesEdits": false
  }]
}

After:

{
  "fixes": [{
    "id": "make-binding-mutable",
    "appliesEdits": false,
    "edits": [{
      "line": 4,
      "old": "    let dst: [4]u8 = [0, 0, 0, 0]",
      "new": "    let mut dst: [4]u8 = [0, 0, 0, 0]"
    }]
  }]
}

Changes

• print_fix_plan_json now takes an optional const SourceInput *input parameter
• Uses existing find_make_binding_mutable_edit to compute the edit preview
• Adds edits array to each fix object when an edit is available
• All three call sites updated to pass &input
• Forward declaration added for find_make_binding_mutable_edit

Testing

• All 9 CLI tests pass (npm run test:zero)
• All 8 docs tests pass (npm run docs:test)
• Command contracts pass (npm run command-contracts:local)
• Manual verification of --plan, --patch, and --apply modes

Closes #6

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