Three small follow-ups to #3145, all observability/documentation —
zero behavior change:

1. src/gui/MainWindow.cpp — comment the asymmetric gating in the
   spotTriggered lambda. The TCI branch suppresses on lock / SWR
   sweep / missing slice because it must mirror AetherSDR's local-
   tune outcome (Log4OM cannot diverge from the radio's actual
   state). The radio branch delegates gating to the firmware via
   `spot trigger`. Without the inline note a future "DRY this up"
   pass could silently regress Log4OM behavior under lock or SWR
   sweep.

2. docs/architecture/tci-receivers.md — note that both
   `clicked_on_spot:` and `rx_clicked_on_spot:` are emitted
   unconditionally for every spot click. #3145 made dual emission
   the v2 protocol baseline (no client-capability negotiation),
   which matters for anyone writing a TCI protocol parser.

3. src/core/TciServer.cpp — qCWarning when notifySpotClicked()
   falls back to slices.first() because the caller passed a null
   slice. The MainWindow path always supplies one (guarded on `s`);
   only wireSpotModel's sliceForPanId(panId) can return nullptr.
   Today that silently mis-attributes rx_clicked_on_spot:'s `trx`
   field to slice 0 — invisible in single-slice testing, broken
   in multi-pan. The warning makes the edge case diagnosable in
   the field.

Principle I (FlexLib is the protocol authority) cited because the
dual `clicked_on_spot:` / `rx_clicked_on_spot:` emission and `trx`
indexing rules trace back to ExpertSDR3/TCI v2 spec authority that
#3145 codified — this commit documents that decision, it does not
revisit it.

Blast radius: risk_score=0.1845 on TciServer::notifySpotClicked
seed (4 high-risk callers: MainWindow::MainWindow,
MainWindow::wirePanadapter, SliceModel::sliceId, RadioModel::slices).
Verified the high-risk callers (MainWindow paths) only reach the
function via the non-null-slice arm and never trigger the new
warning branch; the only path that reaches the fallback is the
in-file wireSpotModel lambda when sliceForPanId returns nullptr.
No behavior change for any caller.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>