feat(llm-routing): bound dynamic routing state with a canonical config cache

Description

Part of #536 (per-model routing configuration).

Bound the router's dynamic routing state. Canonicalize each valid expression
for identity (normalized algorithm name, parameters sorted by key) and share
one runtime definition per canonical configuration. Cap dynamic state per
router process: at most 1024 canonical definitions with LRU eviction and a
15 minute idle expiry, with the per-target load-balancer instances created
under those definitions counted against the same budget. In-flight requests
keep the state they started with; the first later request whose auth result
carries an update uses the new configuration.

This ticket makes expressions production-ready: stateful algorithms retain
their per-target state across requests because identical configurations reuse
one definition.

Definition of Done

• Identical expressions (regardless of parameter order or quoting style)
  resolve to one shared cached definition
• Cache respects the entry budget, LRU eviction, and idle expiry; target
  cardinality cannot bypass the budget
• Updating a model's expression requires no reload; in-flight requests are
  isolated from the update
• Tests cover canonical identity, bounds, eviction, no-reload updates, and
  state isolation

Resources

• Parent: #536
• Blocked by: router expression parsing and overlay