The backend's list ends at a whole-program compiler, not a dev one

NEXT.md

@@ -28,6 +28,38 @@ spelled out and it is the one place it is not.
 The fix is a function in `runtime/` — that directory was held by another lane when this landed, which
 is the whole reason it was left. Say what the caller promised and what was passed.
 
+## The step after the backend's list, and it is the one the backend exists for
+
+**Named here because it is not on the list below and would otherwise be discovered late.**
+
+Finishing items 1-6 below gives a second **whole-program** compiler. `--x86` is refused with `--dev`
+today, deliberately, and `DISCUSS.md` item 16 says it has no cells. So at the end of that list the
+backend can build `sand.flan` and cannot serve a single `C-c C-c`.
+
+What the backend was for is `DISCUSS.md` item 13's measurement: `llc` and `ld` are **about 21ms of a
+35ms redefinition**, and removing them is the only thing that touches that number. Collecting it means
+emitting a **redefinition module** — one function, compiled and installed into a running process
+through an indirection cell — which is what `Build.shared` does on the LLVM path.
+
+Concretely, and none of it is scoped yet:
+
+- **`Fnval`'s indirection cell.** Item 4 below. `FnAddr (Fnval n)` emits the symbol, which is right
+  for a whole-program build and wrong the instant anything is redefined into it. This is the
+  prerequisite and it is already on the list; what follows is not.
+- **A module rather than a program.** `Build.shared` compiles one function to a `.so` that
+  `flan_dev.c` loads. The x86 path writes an assembly file for a whole program; emitting a relocatable
+  object for one function, with the right symbols exported and the cells left external, is different
+  work.
+- **The dev registry.** A run-time-new name needs `flan_dev_cell`. `Emit.redefinition` already
+  declares 256 siblings for an imported package and that cost was measured at +3.6ms per eval; the
+  x86 path has no equivalent.
+- **DWARF.** `flan dev --debug` exists and `--x86` refuses `--debug`. A dev backend that cannot be
+  debugged is a narrower tool than the one it replaces.
+- **Then measure, because the whole case rests on it.** If a hand-written backend does not actually
+  beat 35ms end to end, the argument for it was wrong and that is worth knowing. `DISCUSS.md` item 13
+  says the backend is decided at step 4 on a measurement taken at step 3, and this is that
+  measurement finally being takeable.
+
 ## The backend, in the order that finishes it
 
 `DISCUSS.md` item 16 is the report and its verdict is the sequencing: *"the wiring is done and it was