refactor: complexity cleanup across protocol state machines, body pumps, and stages (+3 bug fixes)

[st0o0]

Summary

Audit-driven complexity cleanup across the protocol layer (H1.0/H1.1/H2/H3), body pumps, and
connection stages. Net effect: dead code removed, duplicated lifecycle/teardown logic collapsed
into single entry points, and implicit boolean-flag state replaced with explicit state enums.
Includes three bug fixes found during the audit. No public API or wire-format changes.

Bug fixes

• lifecycle: stop surviving listeners when a sibling listener dies during startup (ServerSupervisorActor)
• body: refund the reserved connection window when a body read fails (FlowControlledBodyPump)
• pooling: allow Return before the first Rent registers a pool factory (ConnectionObjectPool)

Refactorings

New shared components (extracted from duplicated per-protocol code)

• ConnectionRateGuard / ReconnectPolicy — shared rate-guard and reconnect logic, previously duplicated across H1.0/H1.1 client and server state machines
• Http3OutboundWriter — single owner of H3 framing and pump credit, extracted from client/server session managers
• PumpSlotLifecycle — deduplicated slot lifecycle shared by Serial/Multiplexed/FlowControlled body pumps, with unified cancel discipline

State made explicit

• H1.1 client connection lifecycle modeled as an explicit state enum (was scattered boolean flags)
• H2 stream lifecycle as explicit state with a single ReleaseStream teardown path

Decomposition of oversized methods

• H2/H3 EncodeRequest: connection bootstrap and body-send strategy ladder extracted
• H1.1 DecodeClientData / OnResponse: streaming-resume, request-dispatch, and drain-scheduling helpers extracted; response-completion epilogue and keep-alive rearm unified
• H3 StreamManager: body-dispatch and FIN epilogues extracted; headers-frame triage and decode error handling consolidated
• ApplicationBridgeStage / connection stage logics: per-request bookkeeping consolidated into RequestSlot with a single FinishRequest; drain predicates deduped, completion entry points unified

Dead code removal

• Write-only/dead StreamState members in H2/H3, pass-through TryDecodeEof, dead _streamRunning flag, obsolete BodyReadContinue starvation guard (superseded by force-async reads)

Tests

New specs for the extracted components (ConnectionRateGuardSpec, ReconnectPolicySpec,
Http3OutboundWriterSpec, Http2StreamStateLifecycleSpec, ServerSupervisorActorSpec) plus
coverage for the three bug fixes. Full unit + stage suite passes.