docs(context): the improvise-incremental note opens on the backend step 4 added (improvise-u2le)

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01VdfEGXA7aJ6ftMsCc9PJze

context/code/improvise-incremental.md

@@ -1,10 +1,11 @@
 # `improvise-incremental`: the reasoning
 
-`crates/improvise-incremental` is ADR 009 step 2 (improvise-t8ow, epic improvise-bw4p): the
+`crates/improvise-incremental` began as ADR 009 step 2 (improvise-t8ow, epic improvise-bw4p): the
 plug. It holds the `Derived` trait, `Eager` (today's evaluator behind it) and the
-differential harness every later backend is checked with. There is no incremental backend
-here, the evaluator is untouched and no existing crate changed. The decisions are the ADR's
-(`context/adr/009-incremental-evaluation.md`, clause 2). The evidence for each test is in
+differential harness every backend is checked with, and, from step 4 (improvise-u2le), the
+first backend that keeps state, `Incremental` (section "Step 4" below). The step 2 sections
+describe the trait and the oracle; they predate the backend. The decisions are the ADR's
+(`context/adr/009-incremental-evaluation.md`). The evidence for each test is in
 `context/test-evidence/improvise-incremental.md`.
 
 ## The shape
@@ -18,6 +19,7 @@ here, the evaluator is untouched and no existing crate changed. The decisions ar
 | `probes`, `Probe` | every question worth asking of a model, and a way to put it to any backend |
 | `differential` | runs one edit sequence on a candidate and on `Eager`, comparing every probe at every step |
 | `Divergence`, `Report` | what a failure names (step, probe, both answers) and what a pass compared |
+| `Incremental` | the backend of step 4: maintained groups and occupancy; formulas and `Sum`/`Avg` still go to the model (later steps change that) |
 
 ## Interpretations