fix(tui,coding-agent): reduce render cost of theme changes and streaming

Theme previews re-render the whole transcript, and streaming re-renders
every frame, so per-line and per-session work dominated in long sessions.

- visibleWidth: fast path for ANSI-styled ASCII, allocation-free escape scanning
- Box: compare unpadded child lines and skip a second width measurement
- Markdown: reuse parsed tokens across invalidation
- footer: cache usage totals and context usage per session state
- bash results: cache the collapsed preview including the expand hint
- sanitizeBinaryOutput: single regex replace

packages/tui/test/visible-width.test.ts

@@ -0,0 +1,26 @@
+import assert from "node:assert";
+import { describe, it } from "node:test";
+import { visibleWidth } from "../src/utils.ts";
+
+describe("visibleWidth", () => {
+	it("measures styled ASCII without counting escape sequences", () => {
+		assert.strictEqual(visibleWidth("\x1b[38;5;4mhello\x1b[39m world"), 11);
+		assert.strictEqual(visibleWidth("\x1b]8;;https://example.com\x07link\x1b]8;;\x07"), 4);
+		assert.strictEqual(visibleWidth("\x1b]133;A\x1b\\prompt"), 6);
+		assert.strictEqual(visibleWidth("\x1b_pi:c\x07cursor"), 6);
+	});
+
+	it("counts tabs as three columns in styled text", () => {
+		assert.strictEqual(visibleWidth("\x1b[1ma\tb\x1b[22m"), 5);
+	});
+
+	it("measures styled non-ASCII text", () => {
+		assert.strictEqual(visibleWidth("\x1b[31m日本\x1b[39m ok"), 7);
+		assert.strictEqual(visibleWidth("\x1b[31m─→\x1b[39m"), 2);
+	});
+
+	it("treats unterminated escape sequences as zero-width control characters", () => {
+		assert.strictEqual(visibleWidth("\x1b[31"), 3);
+		assert.strictEqual(visibleWidth("a\x1b"), 1);
+	});
+});