feat(config-settings): add __config_settings_frame shared renderer

The width tier, title-border arithmetic, ON/OFF/DEFAULT badge, cursor cell
and table row are currently hand-copied across the three draw functions.
This adds them once, with the two geometry identities derived rather than
hand-maintained: a row's chrome is a fixed 21 columns, so field_w is
iw - 21 - label_w (reproducing both iw-33 and iw-34), and a title border is
dashes = iw - visible(segment) - 1 (reproducing all three of iw-23,
iw-len-3 and iw-L-S-22).

The frame owns no page height. Every verb prints exactly one line or
fragment, so the fixed-16 category and value pages and the dynamic
7+n sub-category page keep their heights, and config-settings.fish's
erase is unaffected.

No caller yet, so rendering cannot move: the golden's existing page
section is byte-identical (verified with cmp -n over its previous size)
and the file only gains frame-verb cases appended after it.

tests/golden/config-settings-render.txt

@@ -4938,3 +4938,57 @@
          │──────────────────────────────────────────────────│
          │ type value   Enter save   Esc cancel   ⌫ delete  │
          └──────────────────────────────────────────────────┘
+### frame width cols=100
+76
+<END>
+### frame width cols=88
+72
+<END>
+### frame width cols=84
+68
+<END>
+### frame width cols=70
+50
+<END>
+### frame badge onoff val=on
+     ON<END>
+### frame badge onoff val=off
+OFF    <END>
+### frame badge onoff val=DEFAULT
+DEFAULT<END>
+### frame badge onoff val=
+DEFAULT<END>
+### frame badge boolean val=true
+     ON<END>
+### frame badge boolean val=false
+OFF    <END>
+### frame badge boolean val=DEFAULT
+DEFAULT<END>
+### frame cursor hit
+▶ <END>
+### frame cursor miss
+  <END>
+### frame title toggle-page
+           ┌─ Opinionated Settings ─────────────────────────────────────────────────────┐
+<END>
+### frame title subcat-page
+           ┌─ Sub-categories: aliases (Universal) ──────────────────────────────────────┐
+<END>
+### frame title value-page
+           ┌─ Sponge Settings ──────────────────────────────────────────────────────────┐
+<END>
+### frame row pad lw=12
+           │  ▶ Aliases      [      ON ]  cmd shadows                                   │
+<END>
+### frame row cut lw=13
+           │    Notifications [      ON ]  done, WakaTime hook                          │
+<END>
+### frame row cut lw=13 overlong
+           │  ▶ Notifications [      ON ]  ls, cat, cd, du,   │
+<END>
+### frame row shorten lw=12
+           │  ▶ Log dir      [      ON ]  /home/tester/very/long/scrollback/history/…   │
+<END>
+### frame row shorten lw=12 empty
+           │    Log max      [      ON ]                      │
+<END>