refactor(sessions): extract CurrentTurnScope + drop dead _approvalTurnState (superseded by #1508)

[Aaronontheweb]

Continues the LlmSessionActor decomposition started in #1496 (transient-state handlers, steps 1–4). This is step 5 plus an enabling cleanup. Two commits of substance, no wire/behavior change — every touched field is transient (rebuilt on recovery), and no persisted event/snapshot shape changes.

1. Remove the dead _approvalTurnState breadcrumb

_approvalTurnState and its five-subtype ApprovalTurnState hierarchy were write-only: a full-repo grep confirmed no subtype payload was ever read to make a decision, and no test referenced it. Every reference was a self-referential read (used only to build the next breadcrumb value) or a pure write; its only live side effect was a _currentTurnContext write piggybacked inside the mutators.

Removed the field and the hierarchy, preserving every _currentTurnContext write (load-bearing, especially the recovery-time restore in ApplyToolApprovalRequested). ClearApprovalTurnState became ClearCurrentTurnContext. Removing the breadcrumb also exposed two genuinely dead calls: MarkApprovalRedrive only re-set _currentTurnContext one line after it was already set, and MarkApprovalRunningAfterRedrive was a no-op on the normal tool-completion path.

2. Extract CurrentTurnScope (step 5)

Grouped the seven ambient "what turn is active and where did it come from" fields — _currentTurnSource, _currentTurnContext, _currentTrustContext, _activeRecall, _activeTurnId, _activeMessageId, _activeChannelType (~40 read sites) — into one transient handler, Sessions/Handlers/CurrentTurnScope, mirroring the step 1–4 container pattern (DiscoveredToolCache, TurnStateTracker).

• The correlation identity (TurnId/MessageId/ChannelType) is mutated only via Bind(MessageSource?) / Bind(TurnContext), which absorb the former BindTurnTelemetry derivation (including the source-turn-id → message-id → generated-id fallback) and keep the three in lockstep.
• The actor keeps the CrashContextSnapshot breadcrumb (it uses the actor-owned session id + clock) and the read-only audience/boundary precedence projections + trust derivation, reading _turn.* at the call site.
• Reset semantics preserved exactly: only TurnContext is nulled at a turn boundary (the others are overwritten each turn) — no blanket Clear() that would change behavior.

Net: the actor loses 8 instance fields (the 7 above + _approvalTurnState). Adds CurrentTurnScopeTests characterizing the turn-id derivation fallback that BindTurnTelemetry previously had no direct coverage for.

A follow-up commit reuses _turn.TurnId directly in ContinueIncomingUserMessage instead of re-generating it via a now-dead ?? new TurnId(IdGen.ShortId()) fallback (surfaced by review — the extraction is what makes the duplicate path obvious).

Verification

• Full Netclaw.Actors.Tests Sessions suite green (incl. ApprovalRehydrationTests ×19, the primary net for the preserved _currentTurnContext writes, and CompactionIntegrationTests).
• dotnet slopwatch analyze: no new violations. Copyright headers present.
• Not triggered (confirmed): OpenSpec, eval suite, system-skills sync, config-schema, smoke tapes — pure transient-state refactor.

[Aaronontheweb]

Superseded by #1508.

Dropping the CurrentTurnScope extraction: it added a layer of indirection (a new type + a _turn. hop at ~40 call sites) without improving type safety, coherence, or simplification — an anemic container with no behavior or invariant, unlike the step 1–4 handlers it was modeled on. The _approvalTurnState dead-code removal that this PR also contained is unambiguously good and is carried forward on its own in #1508.