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/config-settings-render.fish

@@ -178,6 +178,17 @@ function _cs_render_case --argument-names label panel_h
     command rm -f $t
 end
 
+# ── Frame-verb fragments ──────────────────────────────────────────────────
+# The shared computations pinned directly, not only through the pages that use
+# them. <END> marks the end of each fragment so trailing padding -- which is
+# the entire point of a 7-column badge or a 2-column cursor cell -- shows up in
+# a diff instead of being invisible whitespace.
+function _cs_frame_case --argument-names label
+    printf '### frame %s\n' $label
+    $argv[2..]
+    printf '<END>\n'
+end
+
 # ── Cases ─────────────────────────────────────────────────────────────────
 # 100 -> IW 76, 88 -> IW 72, 84 -> IW 68, 70 -> IW 50: one COLUMNS value per
 # width tier, each a few columns above its threshold.
@@ -222,4 +233,47 @@ begin
         _cs_render_case "edit page=paths row=2 buf=empty" 16 \
             __config_settings_draw_value 2 paths edit ""
     end
+
+    # Emitted last on purpose: everything above is page output, so the page
+    # section's byte offsets never move when this section grows.
+    for cols in 100 88 84 70
+        set -g COLUMNS $cols
+        _cs_frame_case "width cols=$cols" __config_settings_frame width
+    end
+    set -g COLUMNS 100
+
+    set -l head (set_color --bold cyan)
+    set -l rst (set_color normal)
+    set -l p_test (string repeat -n 11 ' ')
+
+    for v in on off DEFAULT ''
+        _cs_frame_case "badge onoff val=$v" __config_settings_frame badge $v
+    end
+    for v in true false DEFAULT
+        _cs_frame_case "badge boolean val=$v" __config_settings_frame badge $v true false
+    end
+
+    _cs_frame_case "cursor hit" __config_settings_frame cursor 3 3
+    _cs_frame_case "cursor miss" __config_settings_frame cursor 3 4
+
+    _cs_frame_case "title toggle-page" __config_settings_frame title 76 $p_test \
+        "$head Opinionated Settings $rst"
+    _cs_frame_case "title subcat-page" __config_settings_frame title 76 $p_test \
+        "$head Sub-categories: aliases (Universal)$rst "
+    _cs_frame_case "title value-page" __config_settings_frame title 76 $p_test \
+        "$head Sponge Settings$rst "
+
+    set -l badge_on (__config_settings_frame badge on)
+    _cs_frame_case "row pad lw=12" __config_settings_frame row 76 $p_test \
+        (__config_settings_frame cursor 0 0) Aliases 12 $badge_on "cmd shadows" pad
+    _cs_frame_case "row cut lw=13" __config_settings_frame row 76 $p_test \
+        (__config_settings_frame cursor 0 1) Notifications 13 $badge_on "done, WakaTime hook" cut
+    _cs_frame_case "row cut lw=13 overlong" __config_settings_frame row 50 $p_test \
+        (__config_settings_frame cursor 0 0) Notifications 13 $badge_on \
+        "ls, cat, cd, du, mkdir, rm, mv, zoxide" cut
+    _cs_frame_case "row shorten lw=12" __config_settings_frame row 76 $p_test \
+        (__config_settings_frame cursor 0 0) "Log dir" 12 $badge_on \
+        /home/tester/very/long/scrollback/history/directory shorten
+    _cs_frame_case "row shorten lw=12 empty" __config_settings_frame row 50 $p_test \
+        (__config_settings_frame cursor 1 0) "Log max" 12 $badge_on "" shorten
 end >$CS_RENDER_OUT

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>