[HOLD][EuiFormControlLayout] Quieter borders

Part of #9893

Depends on: #9894 (quieter-edge decision)

## Status: HOLD

Do not start until #9894 / epic #9893 settles the quieter-edge approach: **real `1px` border** (optionally quieter color) vs **faux border** (`box-shadow: 0 0 0 1px …`).

## Why hold

Form controls already read theme tokens for rest borders:

- `form.borderColor` → `components.forms.border` → **`colors.borderBasePlain`** (not `euiTheme.border.color` / `borderBaseSubdued`)
- Width is still `border.width.thin` (1px)

So a **color-only** quieting on `borderBasePlain` (or remapping `forms.border`) may propagate to `EuiFormControlLayout` automatically — this issue could then **close as absorbed**.

A **faux-border** system likely needs a shared pattern (Panel + forms), not a one-off on `EuiFormControlLayout`.

## Re-open / keep only if still needed after #9894

- Apply the chosen edge system to forms if it doesn’t land via tokens alone
- **Grouped controls** share one outer edge
- Keep **dividers** within redesigned inputs (appends/prepends)
- Keep existing **focus** design

## Goal (when unblocked)

Quieter default form controls — softer rest edge via the shared system — without a parallel one-off that fights the token / pattern decision.

## Related (not this issue)

Form **corner radius** (`border.radius.control`) is tracked on #9896 — not blocked by the quieter-edge HOLD.

Wait for #9894 and epic #9893 to settle the quieter-edge approach before starting. Then inspect EuiFormControlLayout and the form tokens described here; the work is done only if the chosen shared edge system applies to default and grouped controls, preserves append/prepend dividers and existing focus styling, or is absorbed by the token change.

Written by the indexing model from the issue text.