feat(log): name the process in every log line

[mrcsin]

Problem

Every window is its own process, and all of them write one shared log file. A line did not say which process wrote it, so the windows' lines could not be told apart. Group G (Tasks 15-18) of the window-per-process plan, its last group.

What changed

• Program.cs: the template carries [{ProcessId}] after the timestamp, the logger enriches with Environment.ProcessId, and one constant names the property in both places. No package added.
• overview.md, bench.md, readme.md: every window shares the log file; an event's first line carries the id; the file's own length decides the 5 MB roll and every process moves to the newest file at its first write after it fills (checked against the decompiled Serilog.Sinks.File 7.0.0).
• backlog.md: "Platform audit leftovers", 12 entries, each against a file:line that holds on the tree; two stale bullets removed.
• The plan's acceptance is verified across Groups A-G, and the plan moves to docs/plans/completed/.
• Included on purpose, outside the log scope: PenEditorViewTests and PenGroupsViewTests clicked a control the refresh had not yet enabled when the pool thread lagged (reproduced under load); they now wait for IsEffectivelyEnabled. testing-strategy.md names HeadlessWait as the wait for that pool-to-dispatcher hop. Kept here because the flake puts this PR's CI at risk.

Verification

• dotnet build SemiPlot.slnx: 0 warnings. Unit 1591/1591, integration 138/138.
• Acceptance Evidence greps 4-6 and the backlog grep print nothing; the 16 named tests pass.
• Demo stand: every log line carries [<pid>], five process ids seen in one file. Smoke step 8 (database stop and restart) needs converge re-run on this stand, because a container restarted from the Aspire dashboard is recreated without semiplot_app; the viewer reported exactly that.

Per-task commits before collapse

2695b69 docs(plans): count the backlog entries that landed
97cc9d9 docs(plans): record the log-and-backlog run
b6deb6c refactor(log): address smells findings
9cf5221 test(pen-editor): wait for the refresh to enable input
2fffa59 docs(repo): address review findings
79dbb07 docs(plans): resolve the final doc sweep
39edb53 docs(plans): record acceptance verification
c175c90 docs(plans): move audit leftovers to backlog
c90000a feat(log): name the process in every log line

https://claude.ai/code/session_01PfZJa7neMMKPzycFR6iuhD