Immutable version chain (IFD layered deltas) + one menu on every page

• The source model is now an IFD delta chain (adopting the __Tools team's layered-delta review). src/v0/vM.m.p/ folders each hold only what that version added or changed (shared/** + site/**, with _removes.json for deletions); building version V layers all folders ≤ V. A version's input set is therefore frozen by construction — a shared-code change is a new delta folder, never an edit to a shipped one, so rebuilding can no longer rewrite history. A frozen.json gate pins every version output's sha256 as a drift tripwire (node build.js --pin re-pins after an intentional change). The v0.4.2/v0.4.0 duplicated source trees are gone: the v0.4.2 delta is literally one file.
• Chain epoch: the base folder (v0.3.0) carries shared/ as of this release, back-applied to all three selectable versions so they all speak the new host protocol. Immutability holds from here forward.
• One menu everywhere. Top-level pages (Partners, Library, Dev) now carry the full site menu — section links, page links, the version switcher, and Book a demo — instead of a bare "Back to site". Section links on those pages return to the last version page you were on and scroll to that section (the host forwards the anchor after the frame loads). Back from a top-level page now always returns to a version page, never to another top-level page.
• Frozen pages stay current at runtime. The host posts a nav manifest (versions, pages, current file) into each page after it loads; menus and switchers re-render from it. A version page built today will list versions and pages added years from now — without ever being rebuilt (its inlined data remains as the standalone fallback).
• The jsdom App-Mode smoke suite is now committed (test/smoke.jsdom.js) and runs as a build gate whenever jsdom is installed.