docs(package): record known gaps and correct stale claims

Notes the export verb's overwrite-consent obligation post-append, the append's
extension-divergence behavior, and readFilePayload's 4GiB blind spot; fixes a
stale u8string() reference and marks the 4GiB guard as accepted-unexercised.

src/shell/package/CLAUDE.md

@@ -19,7 +19,7 @@ belong to the verbs.
   `std::filesystem` decodes a narrow path through the runtime ANSI code page (measured
   `GetACP() == 1252`), so a bare `fs::path(std::string)` turns `café.rsbank` into
   `café.rsbank` or fails to open it. Every path a verb hands in or gets back —
-  including `listFolderFileNames`' results, which use `u8string()` and never
+  including `listFolderFileNames`' results, which go through `pathToUtf8()` and never
   `string()` — is UTF-8. `core/util/file_bytes` has the un-converted shape, which is
   why `readFilePayload` reads through this module's own `PackageFileReader` instead.
 - **Atomic package write, to the limit of a rename.** A package accumulates in a
@@ -47,7 +47,14 @@ belong to the verbs.
   writer could win the race against. Collision handling (auto-rename) remains the
   import plan's job upstream. The package writer itself DOES replace an existing
   destination — the export save dialog's own overwrite confirm is the consent — and
-  that asymmetry is deliberate.
+  that asymmetry is deliberate. **Known gap, obligation on the export verb:**
+  `pickPackageSavePath`'s own `.rsbank` re-append (see its Gotcha below) can turn a
+  confirmed path `X` into a write target `X.rsbank` that the dialog never asked about.
+  The export verb MUST re-check `fileStatus()` on the path actually handed to
+  `PackageFileWriter` — after any extension append — and get its own consent if that
+  re-checked path is `Present`; the dialog's confirm only ever covered the pre-append
+  path. Not fixed at this seam: prompting is verb-level UX, and `pickPackageSavePath`
+  has no caller yet, so the gap is latent, not live.
 - **The rollback delete is prune's ONE carve-out, and only HALF of it is structural.**
   The citation and the full discriminator live at `package_rollback.cpp`'s header.
   "Did this call create it" is structural: only exclusively-created paths are
@@ -88,7 +95,13 @@ belong to the verbs.
   an extension from `extension_list` when the user omits one — `pickPackageSavePath`
   re-appends `.rsbank` itself so the returned path is correct regardless of how that
   lands (the superseded Win32 path had `ofn.lpstrDefExt` for this; `GetUserFileName`
-  has no equivalent parameter).
+  has no equivalent parameter). The re-append is suffix-blind: it only skips when the
+  path already ends in `.rsbank`, so a path carrying a DIFFERENT extension gets
+  `.rsbank` appended after it (`mybank.bak` → `mybank.bak.rsbank`), unlike the
+  superseded `ofn.lpstrDefExt`, which appended only when the path had no extension at
+  all. Defensible for a format-locked export, but a real divergence from the old
+  picker's behavior — whoever tests the picker under `[verify — DAW]` should expect
+  the double-extension result on a path that already has one.
 - `pickPackageSavePath`'s `suggestedPath` doubles as the dialog's starting directory
   when it is a full path. The verbs should seed it from the project directory —
   passing a bare name leaves the dialog on REAPER's process working directory, which