docs(sbom): add SBOM reference page + withDocHelp on SBOM dashboard

- docs/sbom.md: what SBOM is, lockfile semantics, 5-level maturity standard,
  gap types A–E, per-ecosystem guidance, Syft OSS tooling, inter-repo task
  communication convention, ingest commands, compliance check commands
- sbom.md: wire withDocHelp(h1, "/docs/sbom") — ? button on page title
- observablehq.config.js: add SBOM entry to Reference nav section

EP-CUST-002 registered: Syft-based comprehensive SBOM generation
Task 5f8cade5 created: [repo:railiance-bootstrap] Add Ansible lockfile

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

dashboard/src/sbom.md

@@ -40,6 +40,12 @@ const isCopyleft  = spdx => spdx && COPYLEFT_KW.some(k => spdx.toUpperCase().inc
 
 # SBOM
 
+```js
+import {withDocHelp} from "./components/doc-overlay.js";
+const _h1 = document.querySelector("#observablehq-main h1");
+if (_h1) { _h1.style.position = "relative"; withDocHelp(_h1, "/docs/sbom"); }
+```
+
 ## Overview
 
 ```js