feat(di): HedgeDefinition in ShieldDefinition for configuration-bound hedging

[thomhurst]

Problem

ShieldDefinition (src/Kevlar.Extensions.DependencyInjection/ShieldDefinition.cs:25-40) binds Timeout, Retry, CircuitBreaker, RateLimit, ConcurrencyLimit, and AttemptTimeout. Hedging cannot be expressed from configuration even though HedgeOptions has only scalar settings (MaxHedgedAttempts, Delay). Applications using AddShield(name, IConfiguration) therefore cannot hedge without dropping to code.

Proposed fix

• HedgeDefinition { MaxHedgedAttempts, Delay } bound at ShieldDefinition.Hedge.
• Fixed pipeline position documented; Retry and Hedge both set is a KevlarConfigurationException at build time.
• Fallback stays out of configuration because it needs a delegate; document that.
• ReloadingShieldOptions covers the new field automatically.

Tests

• Binding from JSON produces the expected Describe() string.
• Validation error when both Retry and Hedge are set.
• Reload picks up a changed hedge delay.