fix(deps): exclude Qt 6.12.0 highlighter refcount regression

[lukisch]

Fresh dependency resolution selects PySide6 6.12.0. In the existing native CI, Linux/Python3.10 and3.11 then abort in main.py's highlightBlock during --self-test with none_dealloc. Exclude exactly6.12.0 in requirements.txt, requirements-build.txt and pyproject.toml, retaining each install path's existing minimum and upper bounds.

Related native evidence: CleanMarkdown run38781911920/job116584023745 installs PySide6+shiboken6.12.0 and reports236 passed/1 failed when the self-test subprocess aborts. The independent product-free QSyntaxHighlighter probe in dev-bricks/CodeBox#4 (head e8c80d978550400f3a11a528134a48c0bf95e181, run38785569217) reproduces the refcount corruption with6.12.0, while6.11.2 completes12000 callbacks with stable None references. CodeBox's separate dependency mitigation also passes all12 native cells. CleanMarkdown still requires its own exact-head CI validation.

Validation: all three PEP508 definitions reject6.12.0 and accept6.11.2; source/build constraints agree and all pre-existing bounds remain. git diff --check passes. No application code, tests, workflow, package version or release changes.

Based directly on main0aea162a697c702032383a554f03912cfc4714f2, independent of the web-injection fix#10. This is a dependency mitigation, not an assertion that every desktop fault is solved. Author model: gpt-6.1-sol (Codex). Independent review pending. Draft only; merges disabled; no deployment or runtime mirror change.

Exact-head CI is now complete: Tests38789604162 SUCCESS in all eight native Windows/Linux Python3.10–3.13 cells plus Flutter. The previously failing Linux3.10 and3.11 jobs install PySide6/shiboken6.11.2 and each complete237 passed, including the settings-preservation self-test, with successful process exit. Source-Platform-Smoke P3 run38789604171 also passes both Linux and macOS; push smoke38789589375 succeeds. No tests disabled, assertions weakened or timeouts extended. Independent review and physical desktop acceptance remain pending; no merge or runtime deployment has occurred.

[ellmos-multi-bot]

Independent review by gpt-6.1-sol (separate instance), exact head 93c3a5c. Full three-file diff read. PEP 508 parsing confirms each definition adds only !=6.12.0 and retains its own pre-existing lower/upper bounds; all reject 6.12.0 and accept 6.11.2. No application/test/workflow/version change. Read actual older failing job 116584023745 (related PR #10 head 10ab7ed): PySide6/shiboken 6.12.0 installed, self-test aborts with none_dealloc in highlightBlock. That function's AST is identical at this head. Read current exact-head run 38789604162: Linux 3.10/3.11 jobs 116610433050 and 116610432729 install Qt/shiboken 6.11.2 and each pass all 237 tests with successful job completion. Locally Windows Python 3.12/Qt 6.11.1: 236 passed, one existing symlink-privilege skip; diff check passed. No blocking finding in this narrowly scoped dependency mitigation. This is not an independent reproduction of the C-extension fault or a guarantee that every later Qt version is safe. Explicit Draft-only/merge-disabled hold preserved; no ready, merge, deployment or physical desktop acceptance claim.