refactor(action): replace bash blocks with zero-dependency Node.js modules

Extract ~450 lines of complex bash from action.yaml into modular,
unit-tested Node.js scripts in lib/. All logic is covered by 26 native
node:test unit tests (node --test, no external dependencies).

Modules added:
- lib/platforms.js   - platform parsing and OCI manifest auto-discovery
- lib/distro.js      - SPDX PURL distro detection (os_name/os_version)
- lib/verifier.js    - Trivy arch mapping and SHA256 verification
- lib/diagnostics.js - CI terminal diagnostic formatters
- lib/install-trivy.js  - Trivy install/verify entrypoint (replaces bash step)
- lib/extract-sboms.js  - SBOM extraction entrypoint (replaces bash step)
- lib/scan-sboms.js     - SBOM scan + diff entrypoint (replaces bash step)

Tests added (all pass, auto-discovered by
ode --test):
- test/platforms.test.js
- test/distro.test.js
- test/verifier.test.js
- test/diagnostics.test.js
- test/orchestration.test.js

action.yaml shrinks from 554 to 107 lines. Inputs are bridged to Node
scripts via INPUT_* env vars so modules are fully decoupled from the
Actions runner and testable in isolation.

test/verifier.test.js

@@ -0,0 +1,39 @@
+const test = require('node:test');
+const assert = require('node:assert/strict');
+const fs = require('node:fs');
+const path = require('node:path');
+const os = require('node:os');
+const crypto = require('node:crypto');
+const { mapArch, parseChecksums, verifyFileSha256 } = require('../lib/verifier.js');
+
+test('mapArch maps uname architecture to Trivy release arch', () => {
+  assert.equal(mapArch('x86_64'), '64bit');
+  assert.equal(mapArch('aarch64'), 'ARM64');
+  assert.equal(mapArch('arm64'), 'ARM64');
+  assert.throws(() => mapArch('mips'), /Unsupported architecture/);
+});
+
+test('parseChecksums parses sha256 checksum lockfile into filename map', () => {
+  const content = `
+    e3b0c44298fc1c149afbf4c8996fb92427ae41e4649b934ca495991b7852b855  trivy_0.74.0_Linux-64bit.tar.gz
+    a1b2c3d4e5f60000000000000000000000000000000000000000000000000000  trivy_0.74.0_Linux-ARM64.tar.gz
+  `;
+
+  const map = parseChecksums(content);
+  assert.equal(map.get('trivy_0.74.0_Linux-64bit.tar.gz'), 'e3b0c44298fc1c149afbf4c8996fb92427ae41e4649b934ca495991b7852b855');
+  assert.equal(map.get('trivy_0.74.0_Linux-ARM64.tar.gz'), 'a1b2c3d4e5f60000000000000000000000000000000000000000000000000000');
+  assert.equal(map.has('nonexistent'), false);
+});
+
+test('verifyFileSha256 verifies actual sha256 hash of a file', () => {
+  const tmpDir = fs.mkdtempSync(path.join(os.tmpdir(), 'cve-test-'));
+  const testFile = path.join(tmpDir, 'test.txt');
+  const fileContent = 'hello world\n';
+  fs.writeFileSync(testFile, fileContent);
+
+  const expectedHash = crypto.createHash('sha256').update(fileContent).digest('hex');
+  assert.equal(verifyFileSha256(testFile, expectedHash), true);
+  assert.equal(verifyFileSha256(testFile, 'wronghash'), false);
+
+  fs.rmSync(tmpDir, { recursive: true, force: true });
+});