/mcd-fill

You are executing MCD-Flow Phase 4: The Filler.

You have been given exactly one file to implement. Every function in this file is a state machine action — it handles one specific transition defined in the statechart.

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Your Inputs (provided in this prompt)

• Full content of the skeleton file to implement
• Full content of all types referenced by this file's CONTRACT blocks
• The statechart transitions relevant to this file (from manifest.statechart.transitions)
• The file path to write the result to
• The registry shard path to write status to (e.g. .mcd/registry/<slug-of-file-path>.json)

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Your Task

Replace every CONTRACT: block with working implementation code. Each function implements exactly one state transition action.

Use the provided types exactly as defined — do not widen, narrow, or redefine them.

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Hard Rules

1. DO NOT change function signatures — parameter names, types, and return types are locked.
2. DO NOT change or remove imports — add an import only if strictly necessary and not already present.
3. DO NOT add new exported functions — implement only what exists in the skeleton.
4. Follow the Logic steps in order — do not skip or reorder.
5. Remove the CONTRACT comment block after implementing — final file must have no CONTRACT comments.
6. Honor the transition boundary — each function handles ONLY what its Transition: line defines. Do not reach into adjacent states or trigger side effects outside the transition scope.

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Implementation Standard

• Handle every error case mentioned in the Logic steps
• Raise/throw the exact error types/messages specified (e.g. throw new Error('EMAIL_TAKEN') / raise EmailTakenError())
• Keep implementations minimal — implement what the CONTRACT says, nothing more
• Do not add logging, metrics, or instrumentation unless the CONTRACT specifies it
• Match the target language's idioms (async syntax, error conventions) based on what the skeleton already shows

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Output

1. Write the complete implemented file to the provided file path.

2. Write your status to the registry shard path provided in the prompt (NOT directly to .mcd/registry.json — the orchestrator merges shards after the DAG level completes, to avoid parallel write corruption):

{
  "path": "<file-path>",
  "status": "filled",
  "hash": null,
  "fillAttempts": <current attempt number>,
  "timestamp": "<ISO>"
}

Timestamp via date -u +"%Y-%m-%dT%H:%M:%SZ". hash stays null — mcd-verify computes it after validation.

Do not explain your implementation. Write the file, write the shard, exit.