feat(aura-cli, aura-engine): use-aware unwired introspection; gang-rule prose; LinComb discovery

Harvest sweep, batch 5 of 6.

- introspect --unwired resolves use refs through the registry store:
  the parse+resolve phase is extracted from graph build's path into the
  shared parse_and_resolve_ops, so a use-bearing document introspects
  identically to how it builds (fetch, C29 gate, label resolution) —
  retiring both the hard miss and the 'content id' mislabeling of
  by-name refs. The engine's subgraph closure contract is unchanged
  (store-freedom binds the engine, not the CLI caller).
- ganging a spliced instance's member path refuses with the rule
  (OpError::GangOfSplicedInstance) instead of the typo-shaped
  no-such-param prose; the leaf-typo case keeps its UnknownParam shape
  under a new sibling pin.
- the --node pending note names the follow-up moves on the current
  surface (build, then introspect --params / --unwired), and the
  authoring guide gains a worked LinComb op-script example
  (args-then-bind order, term[i]/weights[i], verified by building it).

refs #339
refs #341

docs/authoring-guide.md

@@ -334,6 +334,33 @@ re-run records, §3).
 (`sub.value` is tapped here purely to illustrate; any interior output field is
 a legal tap source, and a producer needs no other consumer to be tapped.)
 
+### Worked example: a `LinComb` op-script
+
+`LinComb` is **arg-bearing** (#271): `graph introspect --node LinComb` shows
+only the `arity` arg and the pending note until `args` supplies it — the
+ports (`term[0]`, `term[1]`, …) and params (`weights[0]`, `weights[1]`, …)
+form only once `arity` is real, one pair per term. A minimal weighted blend
+of two sources:
+
+```json
+[
+  {"op": "source", "role": "a", "kind": "F64"},
+  {"op": "source", "role": "b", "kind": "F64"},
+  {"op": "add", "type": "LinComb", "name": "combo",
+   "args": {"arity": "2"}, "bind": {"weights[0]": {"F64": 0.7}}},
+  {"op": "feed", "role": "a", "into": ["combo.term[0]"]},
+  {"op": "feed", "role": "b", "into": ["combo.term[1]"]},
+  {"op": "expose", "from": "combo.value", "as": "blend"}
+]
+```
+
+`args` fixes the arity BEFORE `bind`/`feed` can address `term[i]`/`weights[i]`
+at all — the same closed-args-then-bind order every arg-bearing type follows
+(§0/#271). `weights[0]` is bound to `0.7` here, so it becomes a default
+(#246); the unbound `weights[1]` stays a required axis, appearing in `graph
+introspect --params` as `combo.weights[1]:F64` (alongside
+`combo.weights[0]:F64 default=0.7`).
+
 ### Session anchoring: the `SessionFrankfurt` preset
 
 To anchor logic to the trading session, the closed vocabulary ships a