load shedding: Physics-based concurrency control

[szuecs]

Physics-based concurrency control filter https://github.com/laphilosophia/atrion

ref error based https://opensource.zalando.com/skipper/reference/filters/#admissioncontrol
ref latency based load shedding #2346

[ivan-digital]

Picking this up.

Rough plan: a new physicsShedder filter next to admissionControl, but with latency as a first-class signal so it catches gray failures (slow-but-200 backends) the current one misses. Baseline is learned per-route so there's no manual threshold to tune. Shedding itself mirrors admissionControl: probabilistic 503s, same modes, same Admission-Control header, same Pre/PostProcessor lifecycle. Route-level only.