switchyard_route: session decision floor / no-downgrade across continuous chat

[yehuditkerido]

Context

Parent: #2 · POC: #16 · Related: mid-session anti-thrash follow-up

#16 is decision-only per request and has no session floor. Separate from the “judge dies” failure path: when Switchyard is healthy, a continuous chat can still bounce Strong → Weak if later turns score below threshold.

#3 sketched a durable Redis/KV floor as later graduation work. This issue is the in-process / POC floor first.

Goal

Once a session has routed to Strong, do not silently drop back to Weak on later turns while state is available on the replica.

Proposed work

• Choose a session key (header / conversation id — document the contract).
• In-process floor: committed_tier = max(floor, decision) with TTL.
• Config knobs (enable/disable, TTL) with safe defaults for the POC.
• Demo: multi-turn chat that escalates to Strong and stays there.
• Document loss profile on restart / replica hop (points at durable store as later work).

Out of scope

• Durable cross-replica store (revisit Pluggable durable session-floor store for switchyard_route (Redis/KV adapter) #3 / enhancements).
• Async quality sampling / StageRouter / Custom classifier modes.

Acceptance criteria

• Config + docs for session floor behavior and session key.
• Demo or tests: escalate to Strong, then a “easy” turn still stays Strong.
• Clear docs on what is lost on process restart.

[yehuditkerido]

/assign