v0.5.0-beta: merge-with-key import, prior_author provenance, beta versioning

Merge-with-key: decrypt exported posts using original identity seed, re-create
under current identity with prior_author in BlobHeader for provenance tracking.
Download page restructured with stable (v0.4.4) + beta (v0.5.0-beta) sections.
Version bumped across all crates.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

website/design.html

@@ -39,7 +39,7 @@
 
     <div class="container wide">
         <section>
-            <span class="version-badge">v0.4.4 &mdash; 2026-03-31</span>
+            <span class="version-badge">v0.5.0-beta &mdash; 2026-04-05</span>
             <h1 style="font-size: 2rem; font-weight: 800; letter-spacing: -0.03em; margin-bottom: 0.5rem;">Design Document</h1>
             <p>This is the canonical technical reference for ItsGoin. It describes the vision, the architecture, and the current state of every subsystem &mdash; with full implementation detail. This document is versioned; each update records what changed.</p>
             <div class="card" style="margin-top: 1rem;">