cut the reprocessing plant entry back to what is still its own

Everything it used to specify moved out as the output-group merge generalised
it: the pick is REQ-MAT-OUTPUT-GROUP, the all-outcomes gate REQ-MAT-CYCLE, the
buffers REQ-MAT-OUTPUT-BUFFER, the eligible set REQ-LOCK-OUTPUT-POOL, the
recipe control REQ-BLD-AUTO-RECIPE. Restating them here only invited the two
to drift apart.

What is left is what config cannot say: why the building exists -- the
value-preserving counterpart to smelting scrap down, and the only path to
voidsteel -- plus the one rule that really is specific to it, that reprocessing
recipes take no part in the implicit unlock traversal, which until now was only
implied by REQ-LOCK-IMPLICIT naming the other three building types.

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

docs/requirements.md

@@ -152,7 +152,7 @@ Any ship, module, building, or assembler recipe id that appears in no unlock gro
 - REQ-BLD-MINER: **Miner** (2×2): The player selects which ore type it extracts. Each ore type corresponds to a `recipes.toml [[recipe]]` entry with `building = "miner"`, defining the output item and `duration_seconds`. Every asteroid tile is equivalent for mining — any miner can produce any ore type based solely on its selected recipe. Ore never depletes. Only implicitly unlocked ore-type recipes are available for selection (REQ-LOCK-UI-RECIPE).
 - REQ-BLD-SMELTER: **Smelter** (2×2): Converts ore or scrap into basic materials. Its recipe is selected as any other building's is, except that it also picks one for itself from the first material it is offered (REQ-BLD-AUTO-RECIPE). Inputs, outputs, and rates are defined in `recipes.toml [[recipe]]` entries with `building = "smelter"`.
 - REQ-BLD-ASSEMBLER: **Assembler** (3×3): The player selects a recipe from the config-defined crafting tree. Produces what that recipe produces (REQ-MAT-OUTPUT-GROUP) at the rate defined in the corresponding `recipes.toml [[recipe]]` entry with `building = "assembler"`. Only implicitly unlocked recipes are available for selection (REQ-LOCK-UI-RECIPE).
-- REQ-BLD-REPROCESSING: **Reprocessing Plant** (3×3): Consumes scrap per cycle and produces one of several possible higher-level intermediate products, picked at cycle start. It is the one building whose recipes carry **more than one output group** (REQ-MAT-OUTPUT-GROUP) — that, and nothing about the building itself, is what makes its yield random; its inputs, groups, weights and amounts are defined in `recipes.toml [[recipe]]` entries with `building = "reprocessing_plant"` like any other recipe's. The eligible groups are restricted to those whose items are implicitly unlocked (REQ-LOCK-OUTPUT-POOL). Its output side follows the general rules with no exception: one buffer per item any group can produce (REQ-MAT-OUTPUT-BUFFER), and a cycle starts only when every group would fit (REQ-MAT-CYCLE) — which is also what denies the player a reroll by stalling the output belt. Like the Smelter it picks its recipe from the first material it is offered while none is set (REQ-BLD-AUTO-RECIPE).
+- REQ-BLD-REPROCESSING: **Reprocessing Plant** (3×3): Consumes scrap and returns a higher-tier material chosen by chance, as the value-preserving counterpart to smelting scrap down (REQ-BLD-SMELTER) and the only source of voidsteel. Its inputs, output groups and weights are ordinary recipe config (REQ-MAT-OUTPUT-GROUP) with `building = "reprocessing_plant"`; nothing about its behaviour is specific to the building. Reprocessing recipes take no part in the implicit unlock traversal (REQ-LOCK-IMPLICIT), so what it can yield is governed by REQ-LOCK-OUTPUT-POOL alone.
 - REQ-BLD-AUTO-RECIPE: **Automatic recipe selection.** The Smelter and the Reprocessing Plant (REQ-BLD-SMELTER, REQ-BLD-REPROCESSING) are *auto-recipe buildings*. They carry a selected recipe and are configured exactly as a Miner or Assembler is — the same selection button and dialog (REQ-UI-SELECT-BUTTON), buffers sized for that one recipe alone (REQ-MAT-INPUT-BUFFER, REQ-MAT-OUTPUT-BUFFER), and the same pre-configuration on a construction site (REQ-BLD-SITE-CONFIG). They differ in one respect only:
   - **Selection while none is set.** When such a building has no recipe, the first material offered at any of its input ports that some recipe of its type consumes selects that recipe; the material is then accepted as normal. This holds for every intake path — a belt, splitter or tunnel exit at an input port, and a directly coupled producer (REQ-MAT-DIRECT-COUPLE) — because a building that accepts nothing would otherwise leave a coupled producer stuck at its port forever. The choice is deterministic: input ports are examined in order, and where several recipes of the type consume the offered material the first in config order wins.
   - **No further switching.** Once a recipe is set the building keeps it. It does not switch when its buffers run empty, nor when a material belonging to another of its recipes arrives — that material is simply not an accepted input, exactly as for any other building.