fix: boot sequence lines appearing twice

Two init paths both called runBoot() before sessionStorage was set
(the key was only written on completion, ~1.1s later):
- Private init() at DOMContentLoaded → runBoot()
- lt.init() HTML call on DOMContentLoaded → ltInit() → runBoot()

Both found no session key and started simultaneous setInterval loops
on the same <pre>, each appending the same message → every line twice.

Fix: set the sessionStorage key immediately at the start of runBoot()
so any concurrent second call exits early.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

base.js

@@ -314,6 +314,7 @@
   function runBoot(appName, force) {
     const storageKey = 'lt_booted_' + (appName || 'app');
     if (!force && sessionStorage.getItem(storageKey)) return;
+    sessionStorage.setItem(storageKey, '1'); // Claim the run immediately to block double-init
     const overlay = document.getElementById('lt-boot');
     const pre     = document.getElementById('lt-boot-text');
     if (!overlay || !pre) return;
@@ -359,7 +360,6 @@
           overlay.style.opacity    = '0';
           setTimeout(() => { overlay.style.display = 'none'; overlay.style.opacity = ''; overlay.style.transition = ''; }, 520);
         }, 500);
-        sessionStorage.setItem(storageKey, '1');
       }
     }, 65);
   }