ycb_phoenix_live_view_ca432466a4f91

what a new run launched now would send

Custom Phoenix.LiveView UploadWriter failures (init/1, write_chunk/2, and close/2) do not behave like other upload entry errors. Today a writer failure can drop or hide the entry, leave upload-name registration stuck so a later allow/retry cannot proceed, skip or mishandle progress so the UI never shows {:writer_failure, reason}, treat a server writer failure as a generic client entry error, or otherwise leave the LiveView/upload channels inconsistent — including when writer init fails after the entry is already registered.

Fix writer failures so they are handled like other entry errors end-to-end:
- Retain the failed entry with a visible {:writer_failure, reason} until it is explicitly cancelled (or replaced); do not auto-drop it when the upload channel exits.
- Keep the LiveView process alive; the upload channel should shut down cleanly on writer failure.
- Invoke the upload progress callback with the failed entry so apps can react (including cancelling/disallowing in that callback).
- A failed entry must not be joinable/uploadable again; cancelling the last/failed entry must release the upload name so a new upload can be allowed and succeed.
- Successful sibling entries in the same upload must still be consumable individually while a failed entry remains.
- On the client, when the server reports a writer failure, do not apply the generic entry-error path that clears/errors the entry a second time; leave it pending until cancel removes it from the DOM.
- Writer init failures must report and retain the entry the same way write/close failures do.

Acceptance is behavioral: failed writer entries stay visible with the writer failure until cancelled; LiveView stays connected; cancel + re-allow works; siblings can complete; no crash from writer errors.

Interface contract — the change must expose exactly this surface:
Phoenix.LiveView.UploadConfig.fail_entry/3 (conf, entry_ref, reason): retain the entry, record the error (without duplicating the same error pair), mark the entry so entry_pid is nil and register_entry_upload returns {:error, :disallowed}; unregister_completed_entry must leave a failed entry in place. Upload channel join failures for writer init continue to use %{reason: :writer_error}.

Work only inside this repository checkout. Make the code change the task
describes, keeping the diff focused — no drive-by refactors.

When you are done, leave your changes committed or in the working tree;
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