Diagnostics log: stalls, slow IPC, errors, Explorer probes #322

Owner, 2026-10-07: "implement some robust logging and debugging into the program especially to catch stalls for example in explorer or in general".

Prism's half of the diagnostics log built in the terminal core (Maxaubert/PrismTerminal#140, PR Maxaubert/PrismTerminal#141; design in PrismTerminal's docs/superpowers/specs/2026-10-07-diagnostics-log-design.md, schema in docs/diagnostics.md).

• Main: the core's startDiagnostics (page stalls, main event-loop lag, slow IPC, errors, crashes, an fs canary), with Prism's long-wait channels left out of ipc-slow; window-crashes.log kept and its lines also in the diagnostics log.
• Settings: a Diagnostics page (Detailed logging, Log files, Mark a problem).
• Explorer probes: open-folder with ms and entries, sort-slow, guard-slow, search, folder sizes, the watcher; crumbs for tabs, projects, the player and archive jobs.
• The log is local only, in %APPDATA%\Prism\logs, never sent.