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.

lib/verifier.js

@@ -0,0 +1,56 @@
+/**
+ * Architecture mapping and cryptographic checksum verification.
+ */
+
+const fs = require('node:fs');
+const crypto = require('node:crypto');
+
+function mapArch(unameArch) {
+  const normalized = (unameArch || '').trim().toLowerCase();
+  switch (normalized) {
+    case 'x86_64':
+    case 'amd64':
+      return '64bit';
+    case 'aarch64':
+    case 'arm64':
+      return 'ARM64';
+    default:
+      throw new Error(`Unsupported architecture: ${unameArch}`);
+  }
+}
+
+function parseChecksums(checksumsContent) {
+  const map = new Map();
+  if (!checksumsContent) return map;
+
+  const lines = checksumsContent.split(/\r?\n/);
+  for (const line of lines) {
+    const trimmed = line.trim();
+    if (!trimmed || trimmed.startsWith('#')) continue;
+    // Format: <hash>  <filename>
+    const match = trimmed.match(/^([a-fA-F0-9]{64})\s+(.+)$/);
+    if (match) {
+      const hash = match[1].toLowerCase();
+      const filename = match[2].trim();
+      map.set(filename, hash);
+    }
+  }
+  return map;
+}
+
+function verifyFileSha256(filePath, expectedHash) {
+  if (!fs.existsSync(filePath) || !expectedHash) return false;
+  try {
+    const fileBuffer = fs.readFileSync(filePath);
+    const actualHash = crypto.createHash('sha256').update(fileBuffer).digest('hex');
+    return actualHash.toLowerCase() === expectedHash.toLowerCase();
+  } catch {
+    return false;
+  }
+}
+
+module.exports = {
+  mapArch,
+  parseChecksums,
+  verifyFileSha256,
+};