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/platforms.js

@@ -0,0 +1,54 @@
+/**
+ * Platform and architecture resolution utilities.
+ */
+
+function parsePlatforms(input) {
+  if (!input || typeof input !== 'string') return [];
+  const parts = input.split(',');
+  const platforms = [];
+  for (const part of parts) {
+    const clean = part.trim();
+    if (!clean) continue;
+    if (!clean.includes('/')) {
+      platforms.push(`linux/${clean}`);
+    } else {
+      platforms.push(clean);
+    }
+  }
+  return platforms;
+}
+
+function discoverPlatforms(headRawJson, targetRawJson) {
+  const set = new Set();
+  for (const raw of [headRawJson, targetRawJson]) {
+    if (!raw) continue;
+    try {
+      const data = JSON.parse(raw);
+      const manifests = data.manifests || [];
+      for (const m of manifests) {
+        if (m.platform && m.platform.architecture && m.platform.architecture !== 'unknown') {
+          const os = m.platform.os || 'linux';
+          const variant = m.platform.variant ? `/${m.platform.variant}` : '';
+          set.add(`${os}/${m.platform.architecture}${variant}`);
+        }
+      }
+    } catch {
+      // Ignore JSON parse errors from invalid manifests
+    }
+  }
+  return Array.from(set).sort();
+}
+
+function toArchTag(platform) {
+  if (!platform) return 'default';
+  // Strip optional leading os/ (e.g. linux/)
+  const withoutOs = platform.replace(/^[a-zA-Z0-9_-]+\//, '');
+  // Replace slashes with dashes (e.g. arm/v7 -> arm-v7)
+  return withoutOs.replace(/\//g, '-');
+}
+
+module.exports = {
+  parsePlatforms,
+  discoverPlatforms,
+  toArchTag,
+};