Chart builder: field-first shelf + value-or-field channels

docs/ux-second-pass.md

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+# UX second pass — batched council review
+
+A running parking lot of small UX / interaction decisions deferred for a **batched
+[`/council`](../.claude/skills/council/SKILL.md) review**, rather than gating each one the
+moment it surfaces. Append quirks here as they come up; resolve them together in one pass,
+record the resolution into the contract (`docs/architecture/09`+`10` and the relevant
+`docs/spec/`), then delete the row.
+
+## Open
+
+### Chart Builder — encoding type control: cycle vs. explicit pick
+
+- **Where:** `src/app/components/ChartBuilderModal.tsx` → `ChannelPill` (the `pillType` chip).
+- **What:** a mapped channel's field type is shown as a **cycling chip** — activating it steps
+  N → O → Q → T within the set valid for the column/channel. It is compact and matches the
+  approved field-first pill design, but a _cycling_ button gives a screen-reader / keyboard
+  user no way to jump straight to a type; they step through the cycle. The pre-2B control was
+  an explicit `N | O | Q | T` segmented radio (direct pick, APG radio pattern).
+- **Decision to make:** keep the cycle, restore the explicit segmented control, or a hybrid
+  (chip for a quick mouse cycle + an accessible radio/menu for direct selection).
+- **Lens:** WAI-ARIA APG (radio vs. toggle button), IBM Carbon, the project's keyboard-first
+  accessibility bar.
+
+### Chart Builder — empty Colour/Size "or constant" affordance
+
+- **Where:** `ChannelPill` / `ChannelSlot` — the constant-value entry on Colour/Size (2A).
+- **What:** an empty Colour/Size slot offers a small underlined "or constant" text button to
+  switch the channel from a field binding to a fixed `{ value }`. Confirm the copy, emphasis,
+  and discoverability read well alongside the field-assign affordance.
+- **Lens:** Carbon / GOV.UK content + control emphasis.