perf(story-mode): a page budget that halves the longest live pages

A sweep of the Story core over every local transcript (1,311 files,
3,855 turns: no errors, never more than 12 panels, the slowest page 12
ms) found pages up to 551 KB (157 KB gzipped), refetched every few
seconds while a long turn runs. The page text budget (outputs, hunks,
helper texts; the oldest events give theirs up first) goes from 400,000
to 120,000 characters: 94% of turns never reach it, and the longest page
is now 74 KB gzipped.

An edit whose lines the budget dropped now says so ("left out to keep a
long turn fast"), instead of the wording for a file a script wrote.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

src/web/public/story-mode.js

@@ -3448,7 +3448,9 @@ Object.assign(CodemanApp.prototype, {
           'story-empty',
           edit.hunks?.length
             ? 'Some lines of this edit were left out. Open the file to see all of it.'
-            : 'What changed is not in the transcript: a script or another program wrote this file. Open it to see it as it is now.'
+            : edit.added || edit.removed
+              ? 'The lines of this edit were left out to keep a long turn fast. Open the file to see it as it is now.'
+              : 'What changed is not in the transcript: a script or another program wrote this file. Open it to see it as it is now.'
         )
       );
     return section;