#3606 [Other] setup: --gguf-dir files for Qwen3.6-35B-A3B no longer need a .done mark made by setup

Description

Problem and result:
`START-HERE.bat --family qwen36 --model UD-Q4_K_XL --gguf-dir <folder>` stops with
"<folder>\Qwen3.6-35B-A3B-UD-Q4_K_XL.gguf is missing" when the file is there, with the pinned size, but was
downloaded by another tool (LM Studio here). `install_own_path` treats a file as present only when
`<file>.done` exists, and that mark is written only by setup's own download or by `verify_sha256`, which this
branch never reaches for such a file. The same gate also made the free-disk check ask for the full 22.9 GB on the
drive of the person's own folder.
After this change, with `--gguf-dir` an existing file (and the mmproj) goes to `verify_sha256(..., own=True)`, which
already checks size and SHA-256, leaves the file alone on a mismatch, and writes the mark. A file that is not in the
folder is still reported as missing.

Related discussion: none found. Searched open and closed issues and PRs for "gguf-dir", ".done", "qwen36"
(#3125 is a different Qwen3.6 error; #570 and #92 are about the Flash-Next shard check).

Approach:
One local helper in `install_own_path`: `have(p) = p.exists() and (done(p) or bool(a.gguf_dir))`, used for the
model's disk estimate, the model's download gate and the mmproj's gate. Without `--gguf-dir` nothing changes.

Validation:
  Passed: `python -m unittest tools.test_setup_qwen36 tools.test_setup_ownfiles` (28 tests, OK) on 9de04cd4 plus
    this change, Windows 11, mocked downloads/engine. The new
    `test_gguf_dir_takes_the_persons_own_files_that_have_no_finish_mark` fails without the change (the disk-space
    refusal above) and passes with it. `test_gguf_dir_without_the_file_still_says_it_is_missing` passes.
  Failed: none observed.
  Not run: a full `START-HERE.bat` install after the change (the fix was found with the real command on
    RTX 4000 Ada / Windows 11 and I created the .done marks by hand to continue; the changed run is not yet
    repeated), the other test files, Linux/macOS, AMD.

Limits:
The message "already downloaded" is now also printed for the person's own file; it is checked right after by
`verify_sha256`. Flash-Next's `--gguf-dir` path was not touched.

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