Preserve terminal scrollback when restoring a hidden alternate-screen pane

On Windows, an alternate-screen TUI pane (Claude Code) could lose scroll-up
after being hidden (tab/worktree switch or app backgrounded) and shown again.

Root cause: on hidden->visible restore, applyMainBufferSnapshot
unconditionally wrote a destructive clear (\x1b[2J\x1b[3J\x1b[H) before
replaying the snapshot, wiping xterm's scrollback. The attach/reattach path
already guards this same clear behind !isAlternateScreen (pty-transport.ts);
the snapshot path did not.

Thread an alternateScreen flag from the backend headless serialization
through the pty:getMainBufferSnapshot IPC + preload types into
applyMainBufferSnapshot, and skip the destructive clear when restoring an
alternate-screen buffer. Add a regression test asserting the clear is NOT
emitted for an alt-screen restore (and the existing normal-buffer test still
asserts it IS).

Fixes #5723

src/main/runtime/orca-runtime.ts

@@ -5262,6 +5262,7 @@ export class OrcaRuntimeService {
     seq?: number
     source?: 'headless' | 'renderer'
     oscLinks?: TerminalOscLinkRange[]
+    alternateScreen?: boolean
   } | null> {
     return this.serializeTerminalBufferFromAvailableState(ptyId, opts)
   }
@@ -5278,6 +5279,7 @@ export class OrcaRuntimeService {
     seq?: number
     source?: 'headless' | 'renderer'
     oscLinks?: TerminalOscLinkRange[]
+    alternateScreen?: boolean
   } | null> {
     return this.serializeHeadlessTerminalBuffer(ptyId, { ...opts, includeEmpty: true })
   }
@@ -5294,6 +5296,7 @@ export class OrcaRuntimeService {
     seq?: number
     source?: 'headless' | 'renderer'
     oscLinks?: TerminalOscLinkRange[]
+    alternateScreen?: boolean
   } | null> {
     const headlessSnapshot = await this.serializeHeadlessTerminalBuffer(ptyId, {
       ...opts,
@@ -5540,6 +5543,7 @@ export class OrcaRuntimeService {
     seq?: number
     source?: 'headless' | 'renderer'
     oscLinks?: TerminalOscLinkRange[]
+    alternateScreen?: boolean
   } | null> {
     const headlessSnapshot = await this.serializeHeadlessTerminalBuffer(ptyId, opts)
     if (headlessSnapshot) {
@@ -5650,6 +5654,7 @@ export class OrcaRuntimeService {
     seq?: number
     source?: 'headless'
     oscLinks?: TerminalOscLinkRange[]
+    alternateScreen?: boolean
   } | null> {
     const state = this.headlessTerminals.get(ptyId)
     if (!state) {
@@ -5664,7 +5669,8 @@ export class OrcaRuntimeService {
     // caller can request scrollback so the user can scroll up to see prior
     // agent output.
     const requested = opts.scrollbackRows ?? 0
-    const scrollbackRows = state.emulator.isAlternateScreen ? 0 : requested
+    const isAlternateScreen = state.emulator.isAlternateScreen
+    const scrollbackRows = isAlternateScreen ? 0 : requested
     const snapshot = state.emulator.getSnapshot({ scrollbackRows })
     const data = snapshot.rehydrateSequences + snapshot.snapshotAnsi
     return data.length > 0 || opts.includeEmpty === true
@@ -5676,7 +5682,11 @@ export class OrcaRuntimeService {
           lastTitle: snapshot.lastTitle,
           seq: state.outputSequence,
           source: 'headless',
-          oscLinks: snapshot.oscLinks
+          oscLinks: snapshot.oscLinks,
+          // Why: lets the renderer skip the destructive scrollback clear when
+          // restoring an alt-screen snapshot — clearing wipes xterm's own
+          // history that the TUI relies on for scroll-up after a tab return.
+          alternateScreen: isAlternateScreen
         }
       : null
   }