docs(guide): Session trigger cadence sets emission density, not the index

Ratifies the m37 milestone-fieldtest concern: the guide recommended a
once-per-bar trigger without saying whether bars_since_open depends on
it. It does not — the index is clock-derived (ctx.now()), so a denser
trigger stream (raw m1 price, as the shipped corpus wires it) emits
the same per-bar index more often. The once-per-bar wiring is a
convention for one-emission-per-bar, not a correctness requirement.

docs/authoring-guide.md

@@ -344,6 +344,14 @@ separate in-session bool — `bars_since_open` alone is the contract. DST is
 handled by `chrono-tz`, so the same local minute reads the same index in
 summer (CEST) and winter (CET).
 
+The index is derived from the clock (`ctx.now()`), never counted from trigger
+firings — so the trigger's cadence only sets how *often* the node emits, not
+what it emits. Feeding a denser stream (e.g. a raw m1 `price`) is equally
+valid: the node fires once per input tick and reports the same
+`period_minutes`-bar index throughout that bar. The once-per-bar wiring above
+is the convention when you want exactly one emission per completed bar; it is
+not a correctness requirement.
+
 Wire it like any node —
 `{"op":"add","type":"SessionFrankfurt","bind":{"period_minutes":{"I64":15}}}`,
 feed its `trigger` from a once-per-bar stream, and read `bars_since_open`.