feat(chart): cancel the read a gesture left behind

[mrcsin]

Problem

A drag queries the history of every window it passes. When the gesture ended, a read for a window the operator had already left ran on to its end, up to the 300 s command timeout, and held the next read behind it. A window whose apply threw could not be asked for again.

What changed

Group E (Tasks 9 and 10) of docs/plans/20260929-window-per-process.md.

• The cancellation token runs from IDataProvider.QueryHistoryAsync to the Npgsql reads.
• A paced request during a drag never cancels, so the strip fills while it moves. The trailing request at the end of a gesture cancels the read for any other window. A cancelled read reports nothing.
• The cancel goes through CancelAsync, so Npgsql's blocking cancel request (up to the 15 s connect timeout) never runs on the UI thread or under the debouncer's or Rx's locks.
• The chart is the one owner of the window drawn. A gesture that ends inside it reads nothing and still cancels the read it left. A read for an older pen set opens nothing.
• A window whose apply threw is read again at the next move.
• Two gaps on the same read path are closed. A window starting before the archive's first sample no longer rereads every throttle period. A pen added with older rows reads them once the archive extent widens. Both were also present on master.
• Stepwise input with gaps over 150 ms (wheel notches, key presses) cancels at every step. data-integration.md states this trade-off.

The plan stays in docs/plans/; Groups F and G follow.

Verification

• dotnet test SemiPlot.slnx: 1580 unit and 138 integration tests pass. AReadCancelledMidStatementThrowsInsteadOfFailing cancels a read held by a table lock on the Raw and Minute layers.
• Each review regression has a test that failed before its fix: the cancel on the UI thread and under the gate, the drawn window per pen set, a pen removed and named again, the endless read before the first sample, and the two widen gaps.
• Idle probes: 45 virtual minutes issue no repeated read for any window position.
• Manual stand walk: a 3 s Raw drag fills during the drag and lands within one read, a drag out of the band and back in keeps the window drawn, an idle minute shows no repeated reads in the log, and a close during a drag closes at once. Passed.

Per-task commits before collapse

9f1a414 docs(plans): record the history pipeline exec run
96daf10 fix(chart): read rows a widened archive brings in
ee51051 docs(comments): cut restated history comments
70c98ef fix(chart): stop rereading a window before the first sample
b3c09a9 refactor(chart): address smells findings
1613bf7 fix(chart): read a pen that came back
395c74f fix(chart): keep the drawn window per pen set
685a0d1 fix(chart): address review findings
04d3ec1 fix(chart): request a window again after its apply threw
48b1409 feat(chart): cancel the read a gesture left behind

https://claude.ai/code/session_01PfZJa7neMMKPzycFR6iuhD