feat(realtime): add speed="fast" option and supported_speeds model capability

[AdirAmsalem]

Description

Adds fast mode to the realtime API in the Python SDK.

Fast mode (speed="fast") serves the session from a higher-compute tier for lower latency and higher throughput; output quality is unchanged. It is currently available for lucy-2.5 / lucy-latest and lucy-vton-3.5 / lucy-vton-latest, in the US region only, and is billed at 2x the standard realtime rate for those models. Other models ignore the option. Omit it (the default) for standard mode.

What changes for users:

• RealtimeConnectOptions accepts an optional speed (type RealtimeSpeed, exported from decart and decart.realtime), alongside the existing resolution option. It is kept across automatic reconnects.
• Each model definition advertises the speed tiers it supports via supported_speeds, so callers can check capability before connecting. Passing speed for a model that does not support it emits a one-time warning and the session proceeds in standard mode.
• README, examples README, and the playground (--speed fast) document the option.

Usage

from decart import DecartClient, models
from decart.realtime import RealtimeClient, RealtimeConnectOptions

client = DecartClient(api_key=os.getenv("DECART_API_KEY"))
realtime = await RealtimeClient.connect(
    base_url=client.realtime_base_url,
    api_key=client.api_key,
    local_track=local_track,
    options=RealtimeConnectOptions(
        model=models.realtime("lucy-2.5"),
        on_remote_stream=on_remote_stream,
        speed="fast",  # omit for standard mode
    ),
)

models.realtime("lucy-2.5").supported_speeds  # ("fast",)

Docs: https://docs.platform.decart.ai/sdks/python

Verification

uv sync --all-extras
uv run black --check decart/ tests/ examples/ playground/   -> 41 files would be left unchanged
uv run ruff check decart/ tests/ examples/ playground/      -> All checks passed
uv run mypy decart/                                         -> 37 errors, identical to main (pre-existing; CI runs mypy with continue-on-error)
uv run pytest tests/ -v                                     -> 96 passed
uv run python -m py_compile examples/*.py playground/playground.py -> ok

Live test (production API, 2026-09-23 15:04 UTC)

Ran the SDK's own RealtimeClient.connect path from a sandbox with a synthetic LiveKit video source, held each session ~5s, then disconnected. Every case reached connected then generating and received a remote inference track with ~90 frames in 5s.

Case	Model	speed	Result
Default path	lucy-2.5	unset	connected, no warning
Fast path	lucy-2.5	"fast"	connected, no warning
Fast path	lucy-vton-3.5	"fast"	connected, no warning
Ignored path	lucy-2.1	"fast"	connected; SDK emitted the "does not support speed" warning

Compatibility

Additive and backward compatible: speed defaults to unset, the default connection path is unchanged (pinned by a test), and no model ids, aliases, or defaults change. No version bump.