[nnshah1]

Overview

This PR intentionally changes the current least-in-flight selector. Today least-in-flight ignores regions and each pool's capacity, so a request can leave its region for a pool that has just one fewer request in flight. Least-in-flight now places pools by affinity's rule on both paths (the current selector and the router core plugin):

• nearest region first;
• then lower load, where load is requests in flight over the pool's capacity (from its Relay);
• leave the nearest pool only for one with more than --headroom-factor times its spare room.

Builds on #17989; the diff shows only this PR.

Key Changes

• placement.rs: the placement rule (order, leading_ties, headroom, distance) is now in one place. AffinitySelector, LeastInFlightSelector and the core's LeastInFlightPicker all use it. Affinity's behavior does not change.
• LeastInFlightSelector::new(in_flight, signals, region, headroom_factor): uses the rule above. A pool with no capacity report counts as full, so without --pool-relay requests stay on the nearest pools, fewest in flight first.
• PoolSignals::capacity: requests the pool's workers can run at once, from the freshest Relay report.
• Core: SelectionCorePolicy::LeastInFlight(LeastInFlightConfig { signals, region, headroom_factor }). The directory now also records each pool's region.
• CLI: --router-policy least-in-flight now needs --region, and it follows the Relays (--pool-relay) as affinity does.

Validation

• cargo test --release -p dynamo-global-router -p dynamo-global-frontend -p dynamo-global-common: all pass (common 10, router 29, frontend lib 12 + main 4 + forwarding 14).
• The existing affinity tests pass unchanged against the shared rule.
• New tests:
  • The current selector stays in its region until another region has twice the room, and counts an unknown capacity as full.
  • The core picks the same pool as the current selector on the same counts, capacities and round trips.
  • Argument parsing requires --region for least-in-flight.
• cargo fmt --check and cargo clippy --release --all-targets -- -D warnings are clean.

Issues and Ongoing Work

• Part of DEP DEP: Global Router selects pools through the embedded router core #17638.
• Deployments that run --router-policy least-in-flight without --region now fail to start. Without --pool-relay, no pool's capacity is known, so the rule falls back to "nearest region, then fewest in flight".
• Follow-up: affinity on the core (feat(global-router): session affinity on the router core #17598).

Pull Request Checklist

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✅ Dynamo PR CI passed — run 41869914796 (attempt 1) on e3374fbd79

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