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/distro.js

@@ -0,0 +1,42 @@
+/**
+ * Distribution detection from SPDX SBOM package PURLs.
+ */
+
+function detectDistro(spdxInput) {
+  if (!spdxInput) return null;
+
+  let data = spdxInput;
+  if (typeof spdxInput === 'string' || Buffer.isBuffer(spdxInput)) {
+    try {
+      data = JSON.parse(spdxInput.toString('utf8'));
+    } catch {
+      return null;
+    }
+  }
+
+  if (!data || typeof data !== 'object' || !Array.isArray(data.packages)) {
+    return null;
+  }
+
+  for (const pkg of data.packages) {
+    if (!pkg || !Array.isArray(pkg.externalRefs)) continue;
+    for (const ref of pkg.externalRefs) {
+      if (ref && ref.referenceType === 'purl' && typeof ref.referenceLocator === 'string') {
+        const purl = ref.referenceLocator;
+        const nameMatch = purl.match(/[?&]os_name=([^&]+)/);
+        const verMatch = purl.match(/[?&]os_version=([^&]+)/);
+        if (nameMatch && verMatch) {
+          const osName = decodeURIComponent(nameMatch[1]);
+          const osVer = decodeURIComponent(verMatch[1]);
+          return `${osName}/${osVer}`;
+        }
+      }
+    }
+  }
+
+  return null;
+}
+
+module.exports = {
+  detectDistro,
+};