Add per-route rate-limit overrides on map-form routes

[williamthome]

Description

The rate_limit guard is listener-global, so a cheap health check and an expensive report endpoint share one budget. A rate_limit key on a map-form route now overrides the listener default for that path, with its own bucket namespace so the two do not contend. Routes that declare no override keep using the listener-global limit, and when no route declares one the compiled subset is undefined and the lookup is skipped entirely. Matching reuses the same path and method matchers as dispatch, first-match-wins in declaration order; a path match whose method falls outside the route's allowlist keeps scanning, so an override only applies to the methods that route declares.

Routes = [
    #{path => ~"/health", handler => health_h},
    #{path => ~"/reports", handler => reports_h, methods => [~"POST"],
      rate_limit => #{limit => 5, window => 60_000}}
].

Per-route configs are validated and derived to their {Rate, Cap, Cost} triple at listener init, so a bad config raises there rather than on the request path.

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