feat(cli,docs): ship the ganged open examples + document the gang construct (#61 tasks 7-8)

The two open examples now expose their author-intended single knobs — the
closed builders always bound these pairs to one value by hand; the open forms
falsely offered them as independent axes:
- r_breakout_open: channel_hi.length + channel_lo.length -> channel_length
  (the Donchian channel is structurally ONE parameter)
- r_meanrev_open: mean_window.length + var_window.length -> window; the band
  factor stays an independent axis
Regenerated via the emitters (never hand-edited); the closed examples are
byte-unchanged. The carve builders gang in the open branch only.

Test migration: the two axis-namespace pins, five --real e2e invocations
(single gang axis, the 10,20 diagonal for the campaign pair — the mismatched
20,40/10,20 grid was exactly the configuration space the gang retires), and
the param_stability row counts (4 -> 3). The r-sma walkforward golden anchor
is untouched (it never swept a ganged pair).

Docs: authoring-guide gains the seventh op + the third param state
(open/bound/ganged) + the gang wrap note; README op list + Axis concept;
glossary gang entry; ledger C24 records the gang verb and the pre-ship
Tier-2 dormancy (no format-version bump while no out-of-repo reader exists).

Verified: full workspace suite 1104/0 (--real e2e included, local data
present), clippy -D warnings clean, cargo doc clean; live acceptance:
introspect --params prints channel_length:I64 alone / window:I64 +
band.factor:F64.

closes #61

README.md

@@ -39,7 +39,8 @@ Invoke it as `aura <command> …` (examples below use the plain name).
   a Monte-Carlo seed set, a walk-forward window sequence). Families are persisted
   in a content-addressed store and can be listed, ranked, and reproduced.
 - **Axis** — one named, sweepable knob of a blueprint (e.g. `graph.fast.length`),
-  bound with a comma-separated value list.
+  bound with a comma-separated value list. A **gang** fuses several sibling
+  knobs into one axis (one value drives all members).
 
 ## Running & orchestrating a loaded blueprint
 
@@ -115,7 +116,8 @@ 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`, and `expose`. See
+The op kinds are `source`, `input`, `add`, `feed`, `connect`, `expose`, and
+`gang`. 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.