fix(tui): neutralize terminal controls at display boundary

Model responses, replayed session data, tool presenter output, question metadata, configuration, and diagnostics all cross into ANSI-aware pi-tui renderers. Passing their C0 or C1 controls through unchanged lets an otherwise ordinary transcript emit OSC, CSI, cursor, or title operations in the user terminal.

Introduce one displayText boundary that preserves line-feed layout but renders every other C0/C1 control as visible \\xNN text before application styling is applied. Route transcript blocks, streaming output, tool cards, diffs, plans, dialogs, headers, cwd/title data, notices, errors, and pre-mount startup failures through that boundary while leaving pi-tui and the theme responsible for legitimate terminal control sequences.

Pin the contract at three levels: a settled headless-terminal golden spans the main untrusted display sources, unit coverage checks the pre-fullscreen failure path, and the real Loader/PTY conversation streams hostile OSC, cursor, and C1 probes and proves only their inert textual forms reach the terminal stream.

packages/ui/tui/tests/tui.spec.ts

@@ -856,8 +856,8 @@ describe('terminal mounting', () => {
     ctx.agents.reportStartFailure(AgentId('other'), new Error('other failed'))
     expect(terminal.output).toBe('')
     expect(exit).not.toHaveBeenCalled()
-    ctx.agents.reportStartFailure(AgentId('main'), new Error('resume failed'))
-    expect(terminal.output).toBe('ui-tui: agent "main" failed to start: resume failed\n')
+    ctx.agents.reportStartFailure(AgentId('main'), new Error('resume \u001b]2;failure-controlled\u0007'))
+    expect(terminal.output).toBe('ui-tui: agent "main" failed to start: resume \\x1b]2;failure-controlled\\x07\n')
     expect(exit).toHaveBeenCalledWith(1)
 
     const session = ctx.sessions.create(SessionId('must-not-start'))