docs: add the h3-metal comparison table with a Device column #209

Goal and acceptance

The user's final README request is:

• Add a separate h3-metal section before Standalone speech engines.
• Use the comparison table from the supplied screenshot, preserving its row order, original six columns and all reported values.
• Insert Device as the second column, populated with Apple M5 Max (40-core GPU).
• Keep the original Standalone speech engines heading and entire section unchanged.

Scope and constraints

README-only change. The screenshot matches the official h3-metal table pinned below. Preserve hardware/workload context, cold/warm definitions and averaging methods. Distinguish FastH3's distilled four-step checkpoint from the twenty-step configurations. Imported results are author-reported, without independent benchmark or equal-quality claims.

Ownership and authorization

• Implementation scope accepted through the user's direct request and screenshot in this Codex chat.
• Accountable parent / implementer: Codex.
• Merge, goal acceptance and closure remain with the requesting human.

Inputs and deliverables

• Source: https://github.com/zuiho-kai/h3-metal/blob/53b7fa5d88e2497ab427ab9c7155535f7e6e6817/README.md#performance
• Deliverable: PR docs: add h3-metal comparison table with Device column #210.
• Validation: whitespace check, exact cell-by-cell comparison with the pinned table after removing the added Device column, seven-column/four-row structure, section order, original speech-section equality and independent read-only review.