feat(theme): apply a theme saved by any process

[mrcsin]

Problem

An operator runs several windows, one process each, over one configuration directory. A saved theme took effect only at the next start, and only in the process that saved it.

What changed

Group D (Task 8) of docs/plans/20260929-window-per-process.md.

• Every process watches app/. A change is loaded once per quiet burst, and a changed theme is applied on the UI thread: a theme saved by any window switches all of them within about 2 s. The watch also loads once at start, so a save made while a process was still starting reaches it.
• A buffer overflow reloads. Any other watcher error, or a refused or missing folder, is reported once, and the folder is opened again every 5 s. A failed load reaches the window's message panel and keeps the current theme.
• A theme-only save shows no restart notice. A save that also changes another key says the theme is already applied. An open settings dialog follows the theme its window applied.
• Section files are read, copied and probed with FileShare.ReadWrite | Delete, so one process reading or saving never blocks another process's File.Replace. The lost update between two simultaneous saves is recorded in docs/plans/backlog.md.

The plan stays in docs/plans/; Groups E-G follow. #90 stays open: it is wider than this group.

Verification

• dotnet test SemiPlot.slnx: 1547 unit and 136 integration tests pass. The affected test classes also pass on Linux (mcr.microsoft.com/dotnet/sdk:10.0, non-root): 154 tests. Every review regression has a test that failed before its fix.
• Watcher behaviour was measured on Windows 11 / .NET 10: a deleted folder raises Error, a recreated one raises nothing, and a folder renamed away is followed. File.Replace succeeds against the new share modes and fails against the old ones.
• Manual stand walk: two windows switch theme together with no notice, the open dialog follows the theme, a language save shows the notice and "Restart now", and a window opened by New window outlives the stand. Passed.

Per-task commits before collapse

9d4ac4f docs(theme): record the notice fit and the orphan window's watch
e950dd8 fix(theme): address external review findings
20cbdf8 docs(plans): record the live theme exec run
021f802 fix(theme): report a watcher that stopped
abb1047 docs(comments): cut restated theme test comments
da66d72 fix(theme): address smells findings
d08383f fix(theme): address review findings
99fc145 feat(theme): apply a theme saved by any process

https://claude.ai/code/session_01PfZJa7neMMKPzycFR6iuhD