Reading Library V4.8.3 — Large-file Studio setup repair

Why this update exists
----------------------
V4.8.2 could save the first 22 files of The Headless Horseman Studio, then fail
with NotFoundError while writing file 23: a 4.1 MB PDF at
System/original-texts/Headless_Horseman_Grades_2-3_ORIGINAL_US.pdf.
The exact native Windows cause is not confirmed.

What changed
------------
- Writes Studio assets in 256 KB pieces instead of one large write.
- Automatically retries interrupted file writes up to four times, refreshing
  directory handles before each attempt and aborting incomplete writable streams.
- Verifies the saved length of each Studio file, not just the launch file.
- Remembers a selected valid Library directory BEFORE extraction, so clicking
  Finish Studio setup again does not normally require choosing the folder again.
- Keeps normal ZIP imports, catalogue relationships, reading assessments and
  local data handling unchanged.

Optional Windows fallback
-------------------------
Drag the ORIGINAL purchased Studio ZIP onto:
  Reading Library Files/INSTALL_STUDIO_WINDOWS.cmd
The accompanying System/INSTALL_STUDIO_WINDOWS.ps1 uses Windows PowerShell and
installs it into Reading Library Files/System/studios/<Product ID>/ without
browser folder-permission handling. It validates the Studio manifest and
checks for path traversal. After success, use the 'I installed these files
manually' button in the Studio resource page. Existing ZIP in My Library need
not be reimported.

Installation
------------
Back up your current Reading Library Files folder. Extract the Upgrade Only ZIP
into its parent folder, accepting merge/replace. Reopen START_READING_LIBRARY.html
in Chrome or Edge. Open the Studio card in My Library and select Finish Studio
setup again. Avoid OneDrive/network/protected folders if Windows continues to
block file writes.

Verification
------------
JavaScript syntax checked. Node.js simulated filesystem tests installed all
102 Headless Horseman files, checked the 4.1 MB PDF length, and recovered from
a simulated NotFoundError mid-way through writing that PDF. ZIP package format
and entry hashes checked. Actual Windows filesystem-picker and PowerShell
execution still require a Windows desktop test.
