quest: plan generated inputs per differential case

quest/m0/checks/README.md

@@ -20,3 +20,4 @@ Each child reports its CPU saved, cold and warm, against `origin/main`.
 
 - [A warm gate](/quest/m0/checks/cache.md) - the land gate builds through mbx, as moq's does
 - [Fixtures that merge](/quest/m0/checks/fixture-layout.md) - `lang` splits by module, and `reject`'s snapshots key by item, not line
+- [A case's inputs stay as cases are added](/quest/m0/checks/case-inputs.md) - each differential case's generated inputs are seeded by its name

quest/m0/checks/case-inputs.md

@@ -0,0 +1,24 @@
+# [S] A case's inputs stay as cases are added
+
+## Goal
+
+Each case of a differential harness gets generated inputs of its own, seeded from the
+case's name, so adding, removing, or reordering a case changes no other case's inputs, and
+a new case fails only on its own inputs.
+
+## Plan
+
+Decided with the maintainer on 2026-11-02: m0, since every PR adding a case can now fail on
+another case's inputs, costing a fixer run and another gate run.
+
+`ts/tests/differential.ts` draws its random inputs from one seeded stream, and a harness
+picks the case by leading bytes modulo its case count: `stock`'s first byte, `lang`'s first
+two, and each of `task`'s steps' first. Adding a case shifts which case each random input
+reaches, and the full-width values reach only the first eight ops. In
+[#436](https://github.com/moq-dev/rust2/pull/436), adding a `stock` case sent the input
+`421f00ff...` to `buf::write_all`, which hit the `rt.BytesMut` reserve refusal on a
+zero-capacity `take` that no input had reached before. Generate each case's inputs from a
+stream seeded by its name, behind the `ops` a harness exports, as `lang`'s caller already
+exports one per case.
+
+Public API: none.