[sgl-router] policies_reorg: affinity policy and the resolver affinity group

[sherlockwu]

Motivation

Fourth PR of the engine-selection redesign, on top of #40273. Adds session and routing-key affinity on the new path, including the design's affinity-first group and global session modes.

Modifications

• policies_reorg/affinity.rs: AffinityPolicy { kind: Session | Sticky, admission, store, global, fallback }. Session and sticky differ only in which request key they read and which fallback they get, so they share one implementation. An existing binding among the candidates is reused when it passes admission. Otherwise the fallback chooses among admitted candidates and the choice is bound (AffinityStore::bind keeps a concurrent binding that is still a candidate). In HitRequired mode the policy never falls back or binds, and it reports a binding held by a non-candidate as a rejection so the resolver can preserve it.
• PickRequest gains mode, affinity_enabled, and affinity_key(kind, global, value); keys are stage/kind/scope/value with scope global or the bucket id.
• buckets_reorg.rs: Pool.affinity: Option<Arc<dyn Policy>> adds the affinity group ahead of the size buckets, over the stage pool filtered by each engine's own bucket context limit and SLO rule under slo_first. Pool.preserve_global_binding implements global-preserve: after the affinity group rejects a stranded binding, later groups run with affinity disabled. Bucket mode has no affinity group and scopes keys per bucket; global-rebind has the group and lets a size bucket rebind.
• admission::Admission::delegate runs a nested policy over the admitted candidates at the owner's placement; nested fallbacks use AllowAll.
• factory.rs: Dependencies { affinity: Arc<AffinityStore> }. Session-aware falls back to power-of-two; sticky to its configured fallback kind. The affinity group is enabled for sticky and for session-aware in a global mode when buckets are configured.
• The trait's unused pick_fallback default is gone; fallback() remains the introspection hook.

Metrics for sticky and session outcomes are deferred to the switch PR, where the legacy counters are reconciled.

Checklist

• cargo clippy --locked --workspace --all-targets -- -D warnings clean
• cargo test --lib policies_reorg (8) and cargo test --test component policies_reorg (7) pass

🤖 Generated with Claude Code

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CI States

Latest PR Test (Base): ✅ Run #35410405514
Latest PR Test (Extra): ❌ Run #35410405349
Latest PR Test (AMD ROCm 10): ➖ No AMD PR run found for this commit.

[sherlockwu]

Closing: the series stops at the power-of-two PR (#40271) for now.