Let an optional SearchableSelect be cleared again

[StanBarrows]

Why

SearchableSelect picks from a closed set and only ever emits option.value, so once anything was picked there was no gesture that returned to "nothing chosen" — a trap for nullable fields. flows.codebar hit it on the data-source form: the store-dialog field is nullable all the way down, but a selected dialog could never be unselected. Its sibling controls both already had an exit (native Select renders a selectable placeholder option; Combobox is erasable free text); this closes the gap for the third sibling.

What

• New clearable prop (+ clear-label for the ✕'s accessible name, default "Clear selection"). While a selection is visible, an ✕ appears between the label and the chevron; pressing it clears, keeps the menu closed, and hands focus back to the trigger (the ✕ it was on has just unmounted).
• Clearing emits '', not null — the native Select's placeholder payload. Emitting null would widen the emit type to T | null for every consumer, forcing null-handling onto call sites whose select is not clearable and can never receive it. For T = string sites, T | '' collapses to string: existing handlers type-check unchanged.
• The ✕ is a sibling of the trigger, not a child — the trigger is itself a <button>, and nested buttons are invalid HTML that browsers "repair" by splitting apart. Absolutely positioned into the trigger's right end at full control height; only rendered while a selected option's label is actually on screen.
• Version bumped to 1.21.0. (Heads-up while releasing: tag v1.20.0 ships a manifest saying 1.19.1 — the tagged-without-bump failure mode verify-version documents for v1.17.0, so the next tag must be cut from a merged bump.)

Testing

• New Clearable story pins the loop: clear → model '', placeholder restored, menu closed, focus on trigger, ✕ unmounted; re-pick → ✕ back.
• Full suite: 248 Playwright tests pass; typecheck, lint, build clean.
• Consumed from flows.codebar (dist synced locally): its new browser test picks the auto-selected store dialog, clears it via the ✕, saves, and asserts docuware_store_dialog_id is stored as null.

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