CLAUDE_AX_SCREEN_READER=1 set correctly but screen reader mode doesn't activate (v2.1.237)

### Description
Screen reader mode does not activate despite `CLAUDE_AX_SCREEN_READER` being correctly set to `1` in the shell environment before launching Claude Code. No `[Screen Reader Mode: on via env]` confirmation banner appears at startup, and the TUI renders normally (box-drawing chars, in-place redraws, arrow-key menus) instead of switching to flat linear text.

### Environment
- Claude Code version: 2.1.237
- OS: Linux (kernel 7.0.11-76070011-generic)
- Terminal: GNOME Terminal
- Shell: bash

### Steps to reproduce
1. `export CLAUDE_AX_SCREEN_READER=1` in the shell
2. Start a new `claude` session from that shell
3. Observe startup output and TUI behavior

### Expected behavior
Per docs (https://code.claude.com/docs/en/accessibility): first line printed should be `[Screen Reader Mode: on via env]`, and the interface should render as plain labeled lines (`you:`, `claude:`, `tool:`, etc.) with numbered menus instead of arrow-key selection.

### Actual behavior
No confirmation banner; normal TUI rendering (box characters, animated spinners, arrow-key menus) is used instead, as if the variable were unset.

### Verification done
- Confirmed `CLAUDE_AX_SCREEN_READER` is present with value `1` in the process environment (checked via subprocess inherited env).
- Confirmed `claude --version` reports 2.1.237, well above the documented v2.1.181 minimum.
- Searched existing issues (`CLAUDE_AX_SCREEN_READER`, "screen reader mode") — found no existing report of this exact symptom (var confirmed set + version compatible, mode still not activating).

Start by reproducing the issue with `CLAUDE_AX_SCREEN_READER=1` when launching a new `claude` session, then compare the startup output and TUI behavior with the accessibility documentation. Done means the `[Screen Reader Mode: on via env]` banner appears and the interface uses plain labeled lines and numbered menus instead of normal TUI rendering.

Written by the indexing model from the issue text.