docs(adr): ADR 009 requires self-contained operators and leaves width-hint placement to scheduling (improvise-felo.4)

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

context/adr/009-incremental-evaluation.md

@@ -38,6 +38,11 @@ wasm32-clean and serde-free). It provides:
 - operator state behind one trace interface (look up, read a range, apply a delta), so
   later store-memory work can page or compress it.
 
+**Operators are self-contained.** An operator's state and behaviour depend only on its
+inputs and its own state, never on a shared in-process global or on another operator's
+internals. That keeps the graph distributable: in principle the model could run across a
+cluster of machines. Nothing more is designed for that here.
+
 There are no nested streams, no recursion operator and no timestamps beyond one clock.
 **The formula language is the query language.** What a request asks of the model beyond
 its axes and window (filters, aggregation, derived values) is written in formula text
@@ -356,8 +361,8 @@ process, an edge is a function call.
 
 Every column incrementally maintains a width hint, which optimises grid sizing and
 reduces jitter. The hint is an integrated aggregate: a histogram from display width to
-count, so a retraction is O(log d). Which end of the connection maintains it is open
+count, so a retraction is O(log d). Where in the graph the hint is computed is a
-question 1.
+scheduling choice, made as an optimisation later.
 
 ### 11. Rollback and memory
 
@@ -422,11 +427,4 @@ Step 8's required tests:
 
 ## Open questions
 
-1. **Which end of the connection maintains a column's width hint?** The client end knows
+None.
-   which values are potentially in its viewport, so it can maintain the hint over the
-   values it holds, fed by the deltas it already receives. The host end can maintain it
-   over a whole column, through the graph, including values the client has never held.
-   It would then need the request's hidden rows (clause 7's overlay) and, for numbers, the
-   number format. *Recommendation:* the client end, over the values it holds. The host
-   computes a whole-column hint only where a client asks for one, as a filter operator
-   on its connection (clause 9).