[kartojal]

Add depth=50 to Perps book subscriptions while retaining the 20-level default. Parse omitted, ::20 and ::50 channel suffixes and route simultaneous depths independently. REST book depth choices are unchanged.

Validation: 81 event and local WebSocket tests passed; Ruff lint/format and Pyright passed. Tests ran offline; channel support does not assert production availability.

Public contract: Perps AsyncAPI.

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Note

Low Risk
Experimental perps streaming API extension with backward-compatible defaults; routing changes are scoped to book subscriptions and covered by unit tests.

Overview
Adds optional 50-level Perps order book streaming via PerpsBookSpec(depth=50); default remains 20 levels with no behavior change for existing callers.

Subscription wiring now emits book::{id}::50 when depth is 50 (20-level stays book::{id}), and book event routing is explicit: match_for(PerpsBookSpec) delivers only the subscribed depth—50-level specs ignore 20-level channels, while 20-level specs accept unsuffixed channels and ::20 but not ::50. Incoming frames parse book::{id}, book::{id}::20, and book::{id}::50 via an updated channel regex.

Tests cover subscribe/unsubscribe channel names, independent depth routing, parsing, and invalid depth values.

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