WS: structured subscribe helpers (generic param builder)

[martinkersner]

Closes #170

What

Adds structured WS subscribe helpers driven by the generated registry, keeping the raw-string path fully backward-compatible.

• New pure build_param(param_format, **tokens) -> str in datamaxi/aio/ws.py: assembles a wire subscribe param from keyword tokens whose names ARE the raw tokens in WS_CHANNELS[path]["param"].
• Generic / registry-driven — no per-channel name table. New generated channels get structured helpers for free.
• Token -> kwarg rule: a token that is entirely upper-case is lower-cased (SYMBOL -> symbol); every other token is verbatim (exchange, tokenId, srcQuote, ...). Documented in the docstring.
• Grammar handled: None (no params -> reject kwargs), single SYMBOL (no separator), @/: separated tokens (separator inferred), and a trailing [@...] optional both-or-neither group.
• Wired into Subscription.subscribe/unsubscribe and MarketSubscription.subscribe/unsubscribe via a small _resolve_params guard. market= on ticker stays a control kwarg (not a param token).

Usage

ws.ticker.subscribe(symbol="BTC-USDT", exchange="binance", market="spot")
ws.premium.subscribe(src="binance", tgt="upbit", tokenId="bitcoin",
                     srcQuote="USDT", tgtQuote="KRW", srcMkt="spot", tgtMkt="spot")
ws.forex.subscribe("USD-KRW")  # raw path unchanged

Why

Ergonomic named-arg subscribe without hand-writing per-channel mappings (issue's Option 1). Raw wire-string path preserved.

Decisions / notes

• Chose raw-token kwargs (not friendly source_exchange/token_id) per the maintainer's Option 1 — zero per-channel code.
• Mixing raw positional params + keyword tokens raises ValueError.
• Structured kwargs on a param: None channel raise ValueError.
• Async-only (no sync WS client exists).

Test plan

• python -m pytest tests/test_ws.py -q -> 27 passed (13 pre-existing + 14 new).
• flake8 clean; black clean.
• New tests: build_param per format; SYMBOL->symbol rule; errors (missing required, unknown token, partial optional group, None channel, mixing raw+tokens); round-trip structured == raw wire string; integration structured ws.ticker.subscribe(...) sends BTC-USDT@binance over a fake conn.

Known failures

None.