bench: sharp with as many threads as ImageScript

sharp's default concurrency on glibc Linux without jemalloc, like the
CI runners', is one thread; ImageScript uses up to 8 where they pay
off, so sharp gets the same (min(available parallelism, 8)) and the
method says so.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01BsU2GAqbnFSNaYDBGD1Km2

Author: matmen

bench/libs.js

@@ -3,6 +3,7 @@
 // are ones the library doesn't have.
 import fs from 'node:fs';
 import { createRequire } from 'node:module';
+import os from 'node:os';
 import { fileURLToPath } from 'node:url';
 const require = createRequire(import.meta.url);
 
@@ -11,6 +12,8 @@ const ImageScript = await import(process.env.IMAGESCRIPT === 'local' ? '../dist/
 const ImageScriptV1 = require('imagescript-v1');
 const { Jimp, ResizeStrategy } = await import('jimp');
 const sharp = (await import('sharp')).default;
+// (as many threads as ImageScript uses by default: sharp's default on glibc Linux without jemalloc, like CI's, is one)
+sharp.concurrency(Math.min(os.availableParallelism(), 8));
 const canvas = await import('@napi-rs/canvas');
 const imageJS = await import('image-js');
 const photon = require('@silvia-odwyer/photon-node');

bench/report.js

@@ -152,6 +152,9 @@ ${table(Object.keys(results.sizes).map(k => [k, k]), LIBS, (key, lib) => results
 - Where the library lets you choose, resizing is bilinear (ImageScript 1 only has nearest neighbor), blurring is
   gaussian with a radius or sigma of 5, rotating grows the image to fit. Each library uses its own defaults
   otherwise, e.g. its PNG compression level.
+- Threads: ImageScript uses worker threads where they pay off (the processors available, at most 8), and sharp gets
+  as many (\`sharp.concurrency()\`; its default on glibc Linux without jemalloc, like on CI, is one). The other
+  libraries run on one thread.
 - [tinybench](https://github.com/tinylibs/tinybench), at least 1.5 seconds and 3 runs per measurement, every library
   in its own process.
 - The release workflow runs the benchmark for every release and commits the results, on a GitHub Actions runner;