[editor-core] extract shared pieces for history-ot in editor (#36828)

* [editor-core] share the pieces the editor, real-time and history-v1 all need

Project-level history-OT has the browser, real-time and history-v1 all reason
about the same changes, so the rules they reason with move into the shared
library rather than being reimplemented on each side.

- `origin/editor_origin.js` plus `EDITOR_ORIGIN_KIND` and its schema entry: the
  origin an editor client's change carries, holding the editorId that scopes a
  change's timestamp.
- `change_identity.js`: how a submission is recognised when it comes back, so a
  resend of a change that already landed is deduplicated rather than applied
  twice.
- `rebase.js`: transforming a change onto concurrent ones, in the same order on
  both sides, which is what lets the server acknowledge a change with only the
  version it landed at.
- `text_file_defaults.js`: the doc-vs-file extension lists, now read from here
  by web's settings instead of being spelled out again.
- `blob_string_length.js`: the string length of a blob, used by history-v1's
  blob store and by web when it writes an uploaded file into history.
- `file_tree_diff.js`: file-tree operations to a diff, used by
  project-history's FileTreeDiffGenerator.
- `Blob.EMPTY_HASH`, so callers can answer for the empty blob without storing
  it.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

* [project-history] build the file-tree diff with the shared helper

FileTreeDiffGenerator's hand-rolled walk over add/edit/move/delete operations
becomes a call to `buildFileTreeDiff` from overleaf-editor-core, so the editor
and project-history read a file-tree change the same way. Adds the unit tests
the decaffeinated original never had.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

* [web] read the editable-file lists from overleaf-editor-core

web's settings spelled out the text extensions and editable filenames that
decide whether a file becomes a doc, and every other service that has to make
that call carried its own copy. The lists now live in overleaf-editor-core and
web builds settings.textExtensions and settings.editableFilenames from them,
keeping the ADDITIONAL_TEXT_EXTENSIONS override for Server CE/Pro on top.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

* [history-v1] read a blob's string length from overleaf-editor-core

Two copies of getStringLengthOfFile lived here, the one in recover_zip.js
carrying a TODO asking for exactly this. Both now import the shared
implementation, which the blob store keeps re-exporting for backupArchiver.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

---------

Co-authored-by: Claude Opus 5 <noreply@anthropic.com>
GitOrigin-RevId: 31948332f416f56848e3b0dfb5142a4c9f93f508

libraries/overleaf-editor-core/package.json

@@ -39,6 +39,7 @@
     "diff-match-patch": "overleaf/diff-match-patch#89805f9c671a77a263fc53461acd62aa7498f688",
     "lodash": "^4.18.1",
     "p-map": "^4.0.0",
-    "path-browserify": "^1.0.1"
+    "path-browserify": "^1.0.1",
+    "utf-8-validate": "^5.0.4"
   }
 }