feat: add the TUI responsive layout math

Below 72 columns the preview pane is dropped and rows take two lines, the
constraint the `sc` chooser was built around and the reason it is still usable
on a phone; above it a clamped sidebar carries the list and the preview takes
the rest.

Every region is clamped to a non-negative size, so a 5x5 terminal degrades to a
header instead of handing the renderer negative widths.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

src/tui/tui-layout.ts

@@ -0,0 +1,137 @@
+/**
+ * @fileoverview Pure responsive layout math for the TUI frame.
+ *
+ * One rule decides the shape: below 72 columns (Termius, iPhone portrait) the
+ * preview pane is gone and rows take two lines, which is the constraint the
+ * `sc` chooser was built around and the reason it is still usable on a phone.
+ * Above it, a clamped sidebar carries the session list and the preview takes
+ * the rest.
+ *
+ * Rectangles are 1-based (row 1, column 1 is the top-left cell) because that is
+ * what `ESC [ <row>;<col> H` takes, and every region is clamped to a
+ * non-negative size so a 5x5 terminal degrades instead of producing negative
+ * widths that would crash the renderer.
+ *
+ * @module tui/tui-layout
+ */
+
+import type { TuiConnectionStatus } from './tui-types.js';
+
+/** Width at which the preview pane is dropped and rows become two lines. */
+export const NARROW_BREAKPOINT = 72;
+/** Sidebar clamp: narrower than this and a session name stops being readable. */
+export const SIDEBAR_MIN_WIDTH = 34;
+/** Sidebar clamp: wider than this is wasted on a list of short names. */
+export const SIDEBAR_MAX_WIDTH = 44;
+/** A preview thinner than this shows nothing useful, so the layout goes narrow instead. */
+export const PREVIEW_MIN_WIDTH = 24;
+/** Share of the width the sidebar aims for between the clamps. */
+const SIDEBAR_RATIO = 0.36;
+
+export interface TuiRect {
+  /** 1-based terminal row of the first line. */
+  row: number;
+  /** 1-based terminal column of the first cell. */
+  col: number;
+  width: number;
+  height: number;
+}
+
+export interface TuiLayoutOptions {
+  /**
+   * Reserve one line under the header for the connection banner. The caller
+   * decides with `needsBanner(model.connection)`, so layout stays pure math.
+   */
+  banner?: boolean;
+}
+
+export interface TuiLayout {
+  cols: number;
+  rows: number;
+  /** No preview pane, two-line rows. */
+  narrow: boolean;
+  /** Terminal lines one session row occupies. */
+  rowHeight: 1 | 2;
+  header: TuiRect;
+  /** Connection banner, when the caller asked for one and there was room. */
+  banner: TuiRect | null;
+  /** Everything between header and footer, banner included. */
+  body: TuiRect;
+  /** The session list. */
+  list: TuiRect;
+  /** The one-column rule between list and preview; null in narrow mode. */
+  divider: TuiRect | null;
+  /** The preview pane; null in narrow mode. */
+  preview: TuiRect | null;
+  footer: TuiRect;
+}
+
+/** Which connection states get a banner line under the header. */
+export function needsBanner(connection: TuiConnectionStatus): boolean {
+  return connection !== 'connected';
+}
+
+function clamp(value: number, min: number, max: number): number {
+  return Math.min(max, Math.max(min, value));
+}
+
+/**
+ * Rectangles for one frame at `cols` x `rows`.
+ *
+ * The header always exists; the footer appears from 2 rows up; the body is
+ * whatever is left, which may legitimately be zero lines high.
+ */
+export function computeLayout(cols: number, rows: number, options: TuiLayoutOptions = {}): TuiLayout {
+  const width = Math.max(1, Math.floor(cols) || 1);
+  const height = Math.max(1, Math.floor(rows) || 1);
+
+  const headerHeight = 1;
+  const footerHeight = height >= 2 ? 1 : 0;
+  const bodyHeight = Math.max(0, height - headerHeight - footerHeight);
+  const bodyRow = headerHeight + 1;
+
+  const header: TuiRect = { row: 1, col: 1, width, height: headerHeight };
+  const footer: TuiRect = { row: height, col: 1, width, height: footerHeight };
+  const body: TuiRect = { row: bodyRow, col: 1, width, height: bodyHeight };
+
+  const bannerHeight = options.banner === true && bodyHeight > 0 ? 1 : 0;
+  const banner: TuiRect | null = bannerHeight > 0 ? { row: bodyRow, col: 1, width, height: 1 } : null;
+
+  const contentRow = bodyRow + bannerHeight;
+  const contentHeight = Math.max(0, bodyHeight - bannerHeight);
+
+  const sidebarTarget = Math.floor(width * SIDEBAR_RATIO);
+  const sidebarWidth = clamp(sidebarTarget, SIDEBAR_MIN_WIDTH, SIDEBAR_MAX_WIDTH);
+  const previewWidth = width - sidebarWidth - 1;
+  const narrow = width < NARROW_BREAKPOINT || previewWidth < PREVIEW_MIN_WIDTH;
+
+  if (narrow) {
+    return {
+      cols: width,
+      rows: height,
+      narrow: true,
+      rowHeight: 2,
+      header,
+      banner,
+      body,
+      list: { row: contentRow, col: 1, width, height: contentHeight },
+      divider: null,
+      preview: null,
+      footer,
+    };
+  }
+
+  return {
+    cols: width,
+    rows: height,
+    narrow: false,
+    rowHeight: 1,
+    header,
+    banner,
+    body,
+    list: { row: contentRow, col: 1, width: sidebarWidth, height: contentHeight },
+    divider: { row: contentRow, col: sidebarWidth + 1, width: 1, height: contentHeight },
+    preview: { row: contentRow, col: sidebarWidth + 2, width: previewWidth, height: contentHeight },
+    footer,
+  };
+}