fleet(#1376): FleetPlane generalization + resolver wiring in dispatch() and handleUpgrade() #1378

Acceptance Criteria

• FleetPlane interface carries optional attached?: HubProxy and keeper?: HubProxy alongside existing hub/client/writes
• dispatch() calls resolveAmicodeTarget for engine-armed (!client) machines in fleet mode; routes "local" → local engine, "keeper" → keeper upstream, "attached" → attached upstream
• The client→hub branch (incl. #1261 hub-down 503) is byte-unchanged
• handleUpgrade() applies the same resolver decision for SSE/WebSocket
• Empty/corrupt pointer defaults to local engine (the resolver's fail-safe)
• Existing fleet_client_relay and amicode_service_attachment_pointer test suites still pass

Testing Decisions

Extend fleet_client_relay.test.ts with a peer-branch test: mock an engine-armed, fleet-mode machine with an attached upstream set; assert dispatch() routes /amicode/roster to keeper and other /amicode/* to attached. Verify the client-branch tests remain byte-unchanged. Reuse the existing resolveAmicodeTarget unit tests (they already cover the D3 decision table).

Key Decisions

• Dual-path dispatch. The resolver governs a NEW peer branch; the never-fork client→hub path is preserved byte-intact. The branch activates on !client && routingMode === "fleet" && attached.
• Single slot, not N-keyed. attached is one optional HubProxy, not a Map — YAGNI for H1's singular pointer.

Constraints & Invariants

• shouldProxyAmicodeToHost and the client branch remain byte-unchanged.
• The #1346 additive-invariants gate passes.
• /amicode/fleet/* never proxies (resolver guarantees local for that path).

Prior Art

• The D3 resolver and its unit tests in attachment_pointer.ts.
• The FleetPlane interface and dispatch() in server.ts.
• The fleet_client_relay.test.ts suite.

Source

Part of #1376. References #1342 (multiplexing resolver) as the original slice.