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/diagnostics.js

@@ -0,0 +1,106 @@
+/**
+ * Diagnostic formatting and reporting utilities.
+ */
+
+function formatSbomOverview({
+  headImage,
+  targetImage,
+  headFiles = [],
+  targetFiles = [],
+  headError = null,
+  targetError = null,
+}) {
+  const lines = [
+    '',
+    '================================================================================',
+    '                      SBOM EXTRACTION DIAGNOSTIC OVERVIEW                       ',
+    '================================================================================',
+  ];
+
+  if (headError) {
+    lines.push(`HEAD Image (${headImage}): ❌ FAILED - ${headError}`);
+  } else {
+    lines.push(`HEAD Image (${headImage}): ✅ SUCCESS`);
+    for (const f of headFiles) {
+      lines.push(`  - ${f.name} (${f.size} bytes)`);
+    }
+  }
+
+  if (targetError) {
+    lines.push(`TARGET Baseline Image (${targetImage}): ❌ FAILED - ${targetError}`);
+  } else {
+    lines.push(`TARGET Baseline Image (${targetImage}): ✅ SUCCESS`);
+    for (const f of targetFiles) {
+      lines.push(`  - ${f.name} (${f.size} bytes)`);
+    }
+  }
+
+  lines.push('================================================================================');
+  lines.push('');
+  return lines.join('\n');
+}
+
+function formatScanOverview({
+  headImage,
+  targetImage,
+  targetBranch = 'main',
+  strategy = null,
+  headSummary = [],
+  targetSummary = [],
+}) {
+  const lines = [
+    '',
+    '================================================================================',
+    '                   CVE VULNERABILITY SCAN DIAGNOSTIC OVERVIEW                  ',
+    '================================================================================',
+  ];
+
+  if (strategy) {
+    lines.push(`Scan Strategy: ${strategy}`);
+    lines.push('--------------------------------------------------------------------------------');
+  }
+
+  lines.push(`HEAD Image: ${headImage}`);
+  for (const line of headSummary) {
+    lines.push(line);
+  }
+
+  lines.push('');
+  lines.push(`TARGET Baseline Image: ${targetImage} (branch: '${targetBranch}')`);
+  for (const line of targetSummary) {
+    lines.push(line);
+  }
+
+  lines.push('================================================================================');
+  lines.push('');
+  return lines.join('\n');
+}
+
+function countVulnerabilities(report) {
+  if (!report) return 0;
+  let data = report;
+  if (typeof report === 'string' || Buffer.isBuffer(report)) {
+    try {
+      data = JSON.parse(report.toString('utf8'));
+    } catch {
+      return 0;
+    }
+  }
+  if (!data || typeof data !== 'object') return 0;
+
+  let count = 0;
+  if (Array.isArray(data.Results)) {
+    for (const res of data.Results) {
+      if (res && Array.isArray(res.Vulnerabilities)) {
+        count += res.Vulnerabilities.length;
+      }
+    }
+  }
+  return count;
+}
+
+module.exports = {
+  formatSbomOverview,
+  formatScanOverview,
+  countVulnerabilities,
+};