fix: build image libraries for baseline VPS CPUs

Replace the SIMD-dependent WebAssembly fallback with source-built libvips and Sharp. Pin the libvips archive checksum, use baseline x86-64 compiler flags, retain the native binding and runtime libraries, and check JPEG/PNG/WebP processing before compilation and in the runtime image.

Lint, type checking, and content/security tests passed. Full Docker verification was stopped at the user request; production build and baseline CPU runtime verification remain pending.

tests/sharp-fallback.test.mjs

@@ -1,26 +0,0 @@
-import assert from 'node:assert/strict';
-import { spawnSync } from 'node:child_process';
-import test from 'node:test';
-
-test('image processing falls back to WebAssembly without native CPU support', () => {
-  // Isolate the native-binding simulation from the rest of the test process.
-  const result = spawnSync(process.execPath, ['-e', `
-    const Module = require('node:module');
-    const original = Module._load;
-    Module._load = function (id, ...args) {
-      if (id.endsWith('.node') && !id.includes('wasm32')) {
-        const error = new Error('Simulated unavailable native binding');
-        error.code = 'MODULE_NOT_FOUND';
-        throw error;
-      }
-      return original.call(this, id, ...args);
-    };
-    const sharp = require('sharp');
-    (async () => {
-      const image = await sharp({ create: { width: 8, height: 8, channels: 3, background: '#336699' } }).webp().toBuffer();
-      const metadata = await sharp(image).metadata();
-      if (metadata.format !== 'webp' || metadata.width !== 8 || metadata.height !== 8) throw new Error('Image round trip failed');
-    })().catch(error => { console.error(error); process.exitCode = 1; });
-  `], { encoding: 'utf8', timeout: 30000 });
-  assert.equal(result.status, 0, result.stderr || result.error?.message);
-});