#1806 config: user_args - engine options of the user's own that setup keeps

Description

## Title
Config: "user_args", engine options of the user's own that setup keeps

## Summary
Setup writes `"args"` on every run, so engine options added there by hand are dropped and named (#629). `"user_args"` is a list of the user's own options, e.g. `["--no-prefill-borrow", "--kv-resident", "0"]`. The server applies it over `"args"` when it starts the engine: an option in both takes the `user_args` value, any other is added. Setup never writes the key, so `carry_over` already keeps it. Split out of #1789 (which now only carries the Windows AMD images), as AGENTS.md asks: one change per PR, a new feature opt-in.

## What changed
- **`serve/server.py`:**
  - `user_args_of()`: a list of strings starting with an option, else a config error at start;
  - `merge_user_args()`;
  - `engine_args()` applies it before #1322's `--vision` check, so that check sees the final options.
- **`serve/runconfig.py`:** the web app's Settings read and edit an option in `user_args` when it is there (else `args`, as before).
- **`setup.py`:** the "options it had that this one has not" line points to `user_args`.
- **Docs:** `docs/INSTALL.md` and `docs/DETAILS.md`.

## Extra Notes
- **Default path:** without the key, `engine_args` returns exactly what it did (tested).
- **Tests:** `serve/test_server.py` `UserArgs`: replace and add, bad values, the Settings page. All `serve` and setup test files pass on `main` 61b3fb5d (1156 tests; rebased).

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