Add a persistent LFM2.5 formatter helper for macOS integrations

[seyeong-han]

Summary

Follow-up to #19195 (which adds LFM2.5 export + the shared make lfm_2_5-mlx runner target). This PR adds a persistent LFM2.5 formatter helper for macOS integrations.

Long-lived companion process for the LFM2.5-350M MLX formatter, mirroring the parakeet helper introduced in #18861. Wraps an executorch::extension::llm::TextLLMRunner with the same JSON-line stdin/stdout protocol the macOS ExecuWhisper app already uses for the parakeet ASR helper, so the formatter model can stay loaded and KV-warm across requests.

Wire contract (kProtocolVersion=1):

Requests:

{"type":"format",   "version":1, "request_id":..., "prompt":...,
 "max_new_tokens":..., "temperature":...}
{"type":"shutdown", "version":1}

Responses:

{"type":"ready",   "version":1}
{"type":"status",  "version":1, "request_id":..., "phase":..., "message":...}
{"type":"result",  "version":1, "request_id":..., "text":..., "stdout":..., "stderr":...,
 "tokens_per_second":<opt double>}
{"type":"error",   "version":1, "request_id":<opt>, "message":..., "details":<opt>}

The Swift counterpart lives at ExecuWhisper/Services/FormatterHelperProtocol.swift in meta-llama/internal-llama-cookbook (end-to-end-use-cases/ExecuWhisper).

Build

cd ~/executorch
make lfm_2_5_formatter-mlx

Produces:

cmake-out/examples/models/llama/lfm25_formatter_helper
cmake-out/examples/models/llama/mlx.metallib

The new lfm_2_5_formatter-mlx Make target depends on the existing lfm_2_5-mlx target; the llama-mlx CMake build preset's targets list now includes lfm25_formatter_helper alongside llama_main.

Test plan

• make -n lfm_2_5_formatter-mlx
• make lfm_2_5_formatter-mlx
• Helper boots, emits {"type":"ready","version":1}, accepts format requests, returns result payloads with tokens_per_second, exits cleanly on shutdown.
• End-to-end via macOS ExecuWhisper app (internal-llama-cookbook/end-to-end-use-cases/ExecuWhisper): formatter helper stays KV-warm across dictation chunks; no per-request reload latency.

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/19562

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⏳ 1 Pending, 2 Unrelated Failures

As of commit d93bf75 with merge base 1c11601 ():

BROKEN TRUNK - The following jobs failed but were present on the merge base:

👉 Rebase onto the `viable/strict` branch to avoid these failures

• pull / unittest / windows / windows-job (gh) (trunk failure)
  backends/xnnpack/test/recipes/test_xnnpack_recipes.py::TestXnnpackRecipes::test_8a4w_recipe
• pull / unittest-editable / windows / windows-job (gh) (trunk failure)
  backends/xnnpack/test/recipes/test_xnnpack_recipes.py::TestXnnpackRecipes::test_8a4w_recipe

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