Make the Tests workflow able to fail on Windows, and skip two POSIX-only tests

The Tests workflow reported SUCCESS on Windows while its own log read
"2 failed, 1114 passed". On Windows a multi-line `run:` block executes under
pwsh, where only the last command decides the step's result -- so
`coverage xml` succeeding hid `pytest` failing. Linux and macOS use bash -e
and did catch it, which is why this was invisible. `shell: bash` on that step
fixes it, and the same hazard is fixed in python-ci.yml's install step. An
audit of every Windows-capable multi-line step found no others; the installer
smoke steps are Unix-guarded and already set -euo pipefail.

A check that cannot go red is worse than no check, because it is trusted --
and this one has been trusted while six Windows tests failed underneath it.

Two of those six are now skipped on Windows: they assert POSIX mode 0o600,
which cannot hold where os.chmod only toggles a read-only flag and st_mode
reads back 0o666. Skipped rather than relaxed, because the property is real
and a weakened assertion would answer it falsely. Whether these files -- the
semantic index among them, which holds stored secrets -- are actually
protected from other users on Windows is an ACL question, now recorded.

The remaining four are TUI tests that assume POSIX paths. They are recorded
rather than patched: reproducing them needs a Windows host, and guessing at
path normalization and palette-argument parsing would mean changing tests to
match an assumption rather than a behaviour. They also interact with the
undeclared-textual finding -- those tests SKIP everywhere except Windows, the
one platform they were not written for.

Co-Authored-By: Claude Opus 5 (1M context) <[email protected]>
Claude-Session: https://claude.ai/code/session_01Vtx94nXDmNgXEeThUZP4QN

.github/workflows/python-ci.yml

@@ -116,6 +116,9 @@ jobs:
           restore-keys: |
             ${{ runner.os }}-pip-
       - name: Install dependencies
+        # See tests.yml: on Windows a multi-line block runs under pwsh, where
+        # a failure in any but the last command is silently ignored.
+        shell: bash
         run: |
           python -m pip install --upgrade pip
           pip install --require-hashes -r requirements.lock