refactor(trading-strategies): align --slippage-clamp flag with broker config key

[bennycode]

Why

The backtest CLI declared a no-slippage-clamp boolean and inverted it by hand:

'no-slippage-clamp': {default: false, type: 'boolean'},
// ...
const clampSlippage = !values['no-slippage-clamp'];

So the CLI name, the local constant, and the slippage.clamp key it ultimately feeds were three different spellings of one setting. The flag was named negatively because parseArgs booleans are presence-only — --slippage-clamp=false throws ERR_PARSE_ARGS_INVALID_OPTION_VALUE, and --slippage-clamp false throws ERR_PARSE_ARGS_UNEXPECTED_POSITIONAL — so a default: true option had no way to be switched off.

parseArgs has a built-in answer for exactly this: allowNegative (added in v22.4.0 / v20.16.0).

What

Declare the option under its positive name and let the parser handle negation:

allowNegative: true,
// ...
'slippage-clamp': {default: true, type: 'boolean'},
// ...
const slippageClamp = values['slippage-clamp'];

--no-slippage-clamp is normalized to slippage-clamp: false during tokenization, so nothing downstream inverts anything and there is never a no-slippage-clamp key.

Invocation	Result
(nothing)	clamp: true
--slippage-clamp	clamp: true — previously an unknown-option error
--no-slippage-clamp	clamp: false — unchanged

Also fixed two help-text nits: the usage line omitted the flag entirely, and its options-block entry broke the description column the other five align to.

Compatibility

No breaking change. --no-slippage-clamp behaves exactly as before; --slippage-clamp becomes newly valid.

Verification

Ran against a real backtest at --slippage-rate 0.05 to confirm the flag reaches BrokerMock rather than only the log line:

--slippage-clamp      P&L: -2697.95 USDT   Portfolio: 10000.00 → 7302.05 USDT
--no-slippage-clamp   P&L: -9990.87 USDT   Portfolio: 10000.00 →     9.13 USDT

Default and explicit-on both print Slippage: 5.00% (clamped to candle range); the negated form omits it. oxlint and oxfmt --list-different are clean.