give every recipe one shape: a list of output groups

Implements REQ-MAT-OUTPUT-GROUP. A recipe had two shapes -- outputs produced
together, or outputs of which exactly one happened -- and every rule over them
was written twice, selected by `building == ReprocessingPlant`: sizing a
buffer, deciding whether a cycle fits, resolving what a cycle makes, costing an
item. RecipeDef now holds output groups, each a weight and a list of items, and
a cycle yields exactly one group. One group is the ordinary recipe, so the old
two cases are the same shape with one and with several, and all four rules
collapse to one expression apiece with no building-type test left.

rollReprocessingOutput becomes rollOutputGroup, where a single group returns
without drawing or testing eligibility. That early-out is load-bearing twice
over. Drawing there would consume entropy for every ordinary recipe and shift
every later random outcome; and eligibility must not apply either, since
implicit unlocking is demand-derived, so an ordinary recipe's output can be
producible while nothing yet calls for it -- testing it would stop the building
producing rather than gate a drop. Past the early-out a group is eligible only
when all of its items are unlocked, being produced whole.

Threat follows the recipe's shape rather than the building, and the per-unit
value now divides by the group's amount as well as its odds. That moves no
number today: every item resolved through this path has amount 1, which is why
the threat expectations are untouched.

Config keeps `outputs = [...]` as the single-group form, so only the two
reprocessing recipes change shape. The recipe summary gains "/" between groups
and keeps "+" within one, which also fixes the plant reading as though a cycle
produced all of its items at once.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Ne3mejABZoLWKLh8fgpM3x

src/lib/sim/ProductionRules.h

@@ -64,12 +64,11 @@ bool hasInputsToStart(const GameConfig& config, const Building& b);
 // counts against that material's capacity (REQ-MAT-OUTPUT-EMERGE, REQ-MAT-OUTPUT-BUFFER).
 bool outputBufferHasRoom(const Building& b, const ItemType& type, int itemCount);
 
-// True when every output a cycle of this recipe could produce would fit -- the gate a
-// cycle has to pass before it may start (REQ-MAT-CYCLE). For a deterministic recipe that
-// is its own outputs. A Reprocessing Plant rolls one of its outputs per cycle
-// (REQ-BLD-REPROCESSING), so each possibility is judged on its own and all must fit: the
-// roll is committed the moment the cycle starts, and testing every outcome rather than
-// the rolled one is what keeps a stalled output belt from biasing the distribution.
+// True when every one of the recipe's output groups would fit -- the gate a cycle has to
+// pass before it may start (REQ-MAT-CYCLE, REQ-MAT-OUTPUT-GROUP). With a single group that
+// is simply that group. With several the pick is committed the moment the cycle starts, so
+// every outcome must fit: testing all of them rather than the picked one is what keeps a
+// stalled output belt from biasing the distribution.
 bool recipeOutputsFit(const Building& b, const RecipeDef& recipe);
 
 // True when a production cycle could actually start right now: some candidate recipe