refactor: move fit to size function

docs/architecture-audit-2026-10.md

@@ -91,12 +91,14 @@ aligned, ontogglealign).
 mime-контракт `OutputMime`; сам `io.ts` таблицу не использовал. Потребители (4
 файла) переведены на новый путь.
 
-### M6. Доменная политика сжатия в IO-модуле
+### M6. Доменная политика сжатия в IO-модуле — СДЕЛАНО (2026-10)
 
-`web/src/lib/core/io.ts:191-210`: `fitWithinBytes` — стратегия «бинарный поиск
-по quality / по числу цветов квантизации» поверх `quantizeImage`,
-`findMaxColorsWithin`, `findQualityWithin`. Это доменная логика, не ввод-вывод
-(у файла 19 экспортов).
+`fitWithinBytes` перенесён из `core/io.ts` в `core/compress.ts` (дом стратегий
+сжатия). Сигнатура с инъекцией:
+`fitWithinBytes(img, mime, targetBytes, quality, encodeFn)` — вызывающий
+(`schema-tool-runner`) передаёт `io.encode`, поэтому `compress.ts` остаётся
+DOM-free и node-тестируемым (попутно `fitWithinBytes` теперь можно
+юнит-тестировать с фейковым encode). `io.ts` вернулся к чистому IO.
 
 ### M7. `clamp` продублирован 5 раз — СДЕЛАНО (2026-10)
 

web/src/lib/components/schema/schema-tool-runner.svelte.ts

@@ -1,10 +1,6 @@
+import { fitWithinBytes } from "$lib/core/compress";
 import { ToolError, type ErrorVars } from "$lib/core/errors";
-import {
-	downloadBlob,
-	encode,
-	fitWithinBytes,
-	type OutputMime,
-} from "$lib/core/io";
+import { downloadBlob, encode, type OutputMime } from "$lib/core/io";
 import { outputFormatByMime } from "$lib/output-formats";
 import type { PixelImage } from "$lib/core/types";
 import { execute } from "$lib/executor";
@@ -155,7 +151,7 @@ async function download(s: RunnerState, ctx: SchemaToolRunnerCtx) {
 		const limit = ctx.limitKb();
 		const quality = q !== undefined ? q / 100 : undefined;
 		const blob = limit
-			? await fitWithinBytes(s.result, out.mime, limit * 1024, quality)
+			? await fitWithinBytes(s.result, out.mime, limit * 1024, quality, encode)
 			: await encode(s.result, out.mime, quality);
 		downloadBlob(blob, `${ctx.pageSlug()}.${out.ext}`);
 	} catch (e) {

web/src/lib/core/compress.ts

@@ -1,3 +1,7 @@
+import type { OutputMime } from "./io";
+import { quantizeImage } from "./quantize";
+import type { PixelImage } from "./types";
+
 /** Compression presets: level -> number of quantization colors. */
 export const COMPRESSION_LEVELS = {
 	light: 192,
@@ -80,3 +84,33 @@ export async function findQualityWithin(
 	}
 	return ok;
 }
+
+export type EncodeFn = (
+	img: PixelImage,
+	mime: OutputMime,
+	quality?: number,
+) => Promise<Blob>;
+
+/** Encodes img to fit targetBytes (quality search for lossy, color count for
+ *  PNG; BMP as-is). `encodeFn` injected (core/io.encode): policy stays DOM-free. */
+export async function fitWithinBytes(
+	img: PixelImage,
+	mime: OutputMime,
+	targetBytes: number,
+	quality: number | undefined,
+	encodeFn: EncodeFn,
+): Promise<Blob> {
+	if (mime === "image/bmp") return encodeFn(img, mime);
+	const initial = await encodeFn(img, mime, quality);
+	if (initial.size <= targetBytes) return initial;
+	if (mime === "image/png") {
+		const encodeSize = async (k: number) =>
+			(await encodeFn(quantizeImage(img, k).image, mime)).size;
+		const k = await findMaxColorsWithin(targetBytes, 256, encodeSize);
+		return encodeFn(quantizeImage(img, k).image, mime);
+	}
+	const encodeSize = async (q: number) =>
+		(await encodeFn(img, mime, q / 100)).size;
+	const q = await findQualityWithin(targetBytes, encodeSize);
+	return encodeFn(img, mime, q / 100);
+}

web/src/lib/core/io.ts

@@ -1,7 +1,5 @@
 import { encodeBmpBytes } from "./bmp";
-import { findMaxColorsWithin, findQualityWithin } from "./compress";
 import { ToolError } from "./errors";
-import { quantizeImage } from "./quantize";
 import { type PixelImage } from "./types";
 
 export type OutputMime =
@@ -144,32 +142,6 @@ function canvasToBlob(
 	});
 }
 
-/**
- * Encodes img to fit targetBytes: binary search over quality for lossy
- * formats, over quantization colors for PNG. BMP has no compression knobs --
- * returned as-is (the caller warns the user the limit does not apply).
- */
-export async function fitWithinBytes(
-	img: PixelImage,
-	mime: OutputMime,
-	targetBytes: number,
-	quality?: number,
-): Promise<Blob> {
-	if (mime === "image/bmp") return encode(img, mime);
-	const initial = await encode(img, mime, quality);
-	if (initial.size <= targetBytes) return initial;
-	if (mime === "image/png") {
-		const encodeSize = async (k: number) =>
-			(await encode(quantizeImage(img, k).image, mime)).size;
-		const k = await findMaxColorsWithin(targetBytes, 256, encodeSize);
-		return encode(quantizeImage(img, k).image, mime);
-	}
-	const encodeSize = async (q: number) =>
-		(await encode(img, mime, q / 100)).size;
-	const q = await findQualityWithin(targetBytes, encodeSize);
-	return encode(img, mime, q / 100);
-}
-
 export function downloadBlob(blob: Blob, filename: string): void {
 	const url = URL.createObjectURL(blob);
 	const a = document.createElement("a");