fix(tui): termux-gate scrollback preservation, touch-friendly defaults

Adds a Termux runtime detection helper and gates three TUI defaults on it:

- Skip the startup scrollback clear on Termux so users can review/copy
  earlier output after reopening the app. Desktop keeps the existing
  \x1b[2J\x1b[H\x1b[3J slate (AlternateScreen takes over there anyway).
- Default INLINE_MODE on under Termux: primary-buffer rendering makes
  long-thread review and copy/paste much less fragile when users
  background/foreground the app. Override with HERMES_TUI_INLINE=0/1.
- Default mouse tracking off under Termux so touch selection isn't
  intercepted by terminal mouse protocols. Explicit override via
  HERMES_TUI_MOUSE_TRACKING=0/1; legacy HERMES_TUI_DISABLE_MOUSE still
  works on desktop.

Detection is purely env-based (TERMUX_VERSION or PREFIX path) with an
explicit opt-out HERMES_TUI_TERMUX_MODE=0 for debugging. Non-Termux
platforms keep every existing default.

Co-authored-by: adybag14-cyber <252811164+adybag14-cyber@users.noreply.github.com>

ui-tui/src/lib/termux.ts

@@ -0,0 +1,29 @@
+const TERMUX_PREFIX = '/data/data/com.termux/files/usr'
+
+const truthy = (value?: string) => /^(?:1|true|yes|on)$/i.test(String(value ?? '').trim())
+
+export const isTermuxEnv = (env: NodeJS.ProcessEnv = process.env): boolean => {
+  const prefix = String(env.PREFIX ?? '')
+
+  return Boolean(env.TERMUX_VERSION) || prefix.includes(TERMUX_PREFIX)
+}
+
+/**
+ * Return true when Hermes should enable Termux-focused TUI defaults.
+ *
+ * Defaults to on in Termux, with an explicit opt-out for debugging:
+ *   HERMES_TUI_TERMUX_MODE=0
+ */
+export const isTermuxTuiMode = (env: NodeJS.ProcessEnv = process.env): boolean => {
+  if (!isTermuxEnv(env)) {
+    return false
+  }
+
+  const override = String(env.HERMES_TUI_TERMUX_MODE ?? '').trim().toLowerCase()
+
+  if (override) {
+    return truthy(override)
+  }
+
+  return true
+}