fix(terminal): decouple scrollback demotion (slice C) from the retention-budget switch (C1 review #2)

Per the approved contract each slice reverts behind its own switch: slice C
now requires only the master terminalHiddenViewParking plus its own
terminalHiddenScrollbackDemotion flag. The TTL wakeup timer fires for
demotion candidates even with the budget switch off. No DEFAULT_SETTINGS
entries exist for sibling flags (defaults are the '!== false' optional
pattern), so no explicit defaults are added.

Co-authored-by: Orca <help@stably.ai>

src/renderer/src/components/Terminal.tsx

@@ -993,13 +993,7 @@ function Terminal(): React.JSX.Element | null {
     )
     const retentionTtlEligibleIds = new Set(
       retentionBudgetCandidates
-        .filter(
-          (candidate) =>
-            !candidate.ordinaryParkingCovers &&
-            !candidate.hasPendingSpawnWork &&
-            // Why: exempt worktrees still need the TTL wakeup — it fires their scrollback demotion.
-            (!candidate.hasEvictionExemptTab || terminalScrollbackDemotionEnabled)
-        )
+        .filter((candidate) => !candidate.ordinaryParkingCovers && !candidate.hasPendingSpawnWork)
         .map((candidate) => candidate.worktreeId)
     )
 
@@ -1017,8 +1011,10 @@ function Terminal(): React.JSX.Element | null {
         hiddenSinceMs: candidate.hiddenSinceMs,
         nowMs,
         ...overrides,
-        // Why: only retention candidates wake at the eviction TTL; everyone else keeps the ordinary deadlines.
-        ...(terminalRetentionBudgetEnabled && retentionTtlEligibleIds.has(candidate.worktreeId)
+        // Why: only retention candidates wake at the eviction TTL; everyone else keeps the ordinary
+        // deadlines. Demotion (slice C) needs the wakeup too — it no longer requires slice B's switch.
+        ...((terminalRetentionBudgetEnabled || terminalScrollbackDemotionEnabled) &&
+        retentionTtlEligibleIds.has(candidate.worktreeId)
           ? {
               retentionTtlMs: overrides.retentionTtlMs ?? TERMINAL_HIDDEN_WORKTREE_RETENTION_TTL_MS
             }

src/renderer/src/components/terminal-pane/terminal-hidden-worktree-retention.test.ts

@@ -185,21 +185,25 @@ describe('selectScrollbackDemotedTerminalWorktrees', () => {
     expect(selected).toEqual(new Set(['wt-aged']))
   })
 
-  it('returns empty when any kill switch is off', () => {
+  it('returns empty when the master or demotion switch is off', () => {
     const worktrees = [exemptCandidate('wt-aged', aged)]
     expect(
       selectScrollbackDemotedTerminalWorktrees({ ...base, worktrees, parkingEnabled: false })
     ).toEqual(new Set())
+    expect(
+      selectScrollbackDemotedTerminalWorktrees({ ...base, worktrees, demotionEnabled: false })
+    ).toEqual(new Set())
+  })
+
+  it('demotes independently of the slice-B retention-budget switch', () => {
+    const worktrees = [exemptCandidate('wt-aged', aged)]
     expect(
       selectScrollbackDemotedTerminalWorktrees({
         ...base,
         worktrees,
         retentionBudgetEnabled: false
       })
-    ).toEqual(new Set())
-    expect(
-      selectScrollbackDemotedTerminalWorktrees({ ...base, worktrees, demotionEnabled: false })
-    ).toEqual(new Set())
+    ).toEqual(new Set(['wt-aged']))
   })
 
   it('honors the retentionTtlMs override and stays time-monotone', () => {

src/renderer/src/components/terminal-pane/terminal-hidden-worktree-retention.ts

@@ -106,7 +106,10 @@ export function selectScrollbackDemotedTerminalWorktrees(
     nowMs: number
   } & TerminalColdParkPolicyOverrides
 ): Set<string> {
-  if (!args.parkingEnabled || !args.retentionBudgetEnabled || !args.demotionEnabled) {
+  // Why parkingEnabled still gates: it is the master kill switch — parking
+  // globally off means nothing parks OR demotes. Slice B's budget switch does
+  // not gate slice C; each slice reverts independently (approved contract).
+  if (!args.parkingEnabled || !args.demotionEnabled) {
     return new Set()
   }
   const ttlMs = args.retentionTtlMs ?? TERMINAL_HIDDEN_WORKTREE_RETENTION_TTL_MS