fix(twincat): review fixes for sum-read and diagnostics (#645)

Strict width+kind eligibility for sum-batched scalars (others read singly); latch only on ADS
1793/1794, split 1792/1797/1798 without latching; 32 KiB per-command budget and 500-item ceiling;
short sum response handled per item. Diagnostics: opt-in diagnosticsEnabled (default off), reuse the
probe's ReadState, sample after the host-state transition, JitterMs renamed HeadroomMs, compact
per-device line so many devices fit the 16-entry cap, unavailable-symbol latch cleared on reconnect.
AdminUI: min/max on sum-read max, diagnostics checkbox. Pure planner extracted and unit-tested.

Claude-Session: https://claude.ai/code/session_01Ef2ozxS7QPanGT8uoATbTe

docs/v3/twincat-backlog.md

@@ -48,13 +48,13 @@ reusing the old source NetId). Nothing below that says "live" has been run again
 ## Added by #612 / #645
 
 6. **ADS sum-read (#310) — code DONE, live gate pending.** Opt-in `useSumRead` / `sumReadMaxItems`
-   (see `docs/drivers/TwinCAT.md` § "Sum-read"). Unit-tested against the fake client only. Live gate
+   (strict width+kind eligibility; latch only on ADS 1793/1794; see `docs/drivers/TwinCAT.md` § "Sum-read"). Unit-tested against the fake client only. Live gate
    (read-only, valid on a press): enable on Z33856 / Z33733 (TC2) and Z35474 (TC3), compare a batched poll
    to individual reads value-for-value, and record whether TC2 CE 2.11 answers or refuses the sum command
    (either outcome is a result — a refusal must fall back cleanly). Needs the route above.
 
-7. **Driver diagnostics (#314) — code DONE, live gate pending.** ADS state, app name, online-change
-   count, task cycle time, last exec time and jitter on the `/hosts` diagnostics bag. Live gate: confirm
+7. **Driver diagnostics (#314) — code DONE (opt-in `diagnosticsEnabled`, default off), live gate pending.** ADS state, app name, online-change
+   count, task cycle time, last exec time and headroom on the `/hosts` diagnostics bag. Live gate: confirm
    `TwinCAT_SystemInfoVarList._TaskInfo[1]` resolves on TC2 2.11 (the driver tolerates its absence).
 
 8. **Flat vs VirtualTree browse on TC3 (#612)** — open. Only Z35474 is TC3; read-only symbol-table