docs: SessionFrankfurt output semantics + declared-tap authoring (#285)

Surfaced by the measurement-milestone fieldtest: a downstream author
reaching for session anchoring or declared taps had no public statement of
either — the semantics lived only in node source / the design ledger.

SessionFrankfurt / Session output semantics:
- glossary `session node`: corrected — it claimed three streams
  (`bars_since_open`, `in_session`, `session_open_ts`), but the shipped node
  emits ONE `i64` field `bars_since_open`. Now states the real contract: the
  count of completed bar-periods since the local (tz-aware, DST-correct)
  session open, close-instant indexed (09:15->1, 09:45->3; pre-open <=0), with
  the Frankfurt 09:00 Europe/Berlin open + `period_minutes` knob + the
  value-ignored `trigger`.
- authoring-guide: a "Session anchoring: the SessionFrankfurt preset"
  subsection — schema (trigger / period_minutes / bars_since_open), the
  close-instant multiples, the EqConst gating pattern, DST handling, and the
  add-snippet. Verified against `aura graph introspect --node SessionFrankfurt`.

Declared-tap authoring (C27 / the #284 tap op):
- authoring-guide: the `tap` op-table row (seven ops -> eight) + a worked
  "declaring a measurement tap" example. The example is verified: piping it
  through `aura graph build` emits
  `taps:[{"name":"spread","from":{"node":2,"field":0}}]`.
- design ledger op-list (INDEX.md): a `tap` bullet (seven verbs -> eight);
  also corrected the `expose` "only verb that keeps `as`" claim — `tap` keeps
  it too.
- README op-kind enumeration: adds `tap`.

Docs only; no code change. Both worked examples were run through the built
`aura` binary; the SessionFrankfurt schema was checked against introspect.

closes #285

README.md

@@ -118,8 +118,9 @@ aura graph build < crossover.ops.json > crossover.bp.json
 aura sweep crossover.bp.json --list-axes
 ```
 
-The op kinds are `source`, `input`, `add`, `feed`, `connect`, `expose`, and
-`gang`. See
+The op kinds are `source`, `input`, `add`, `feed`, `connect`, `expose`, `tap`,
+and `gang` (`tap` declares a recorded measurement point on an interior wire —
+see the authoring guide). See
 `aura graph introspect --node <T>` for a type's exact ports and the op-script
 grammar in the design ledger for the full semantics.