docs+install: post-rename docs polish, install script, d2 architecture diagram (#26)

* docs: add sanderling bird artwork to README and docs index

* chore: add one-line install script for macOS and Linux

Detects os/arch, resolves latest (or pre-)release, verifies sha256,
and installs the binary into $HOME/.sanderling/bin.

* docs: use the one-line installer in getting-started

Replaces the broken `<version>` placeholder snippets with the
install.sh one-liner.

* docs: drop filler line under the install one-liner

* docs: rename index heading to Sanderling Manual

* docs(style): adopt JetBrains Mono and uppercase brand mark

* docs(getting-started): use justfile flow for folio sample

* docs(writing-specs): drop 'coming soon' notes for eventually and implies

* docs(inspect): rewrite layout for tabbed state panels and metrics chart

* docs(inspect): add UI screenshot

* docs: add Inspect to sidebar and index

* docs: restore mermaid bootstrap script in page template

* docs(style): constrain article images to content width

* docs(inspect): drop layout prose, keep what the screenshot doesn't show

* docs(architecture): add d2 source for architecture diagram

Replaces the in-page mermaid block with a d2-rendered SVG. Generated
outputs (svg/png) stay out of git; only the .d2 source is checked in.

* build(docs): render d2 diagrams into build/site/_assets/diagrams

* docs(architecture): swap mermaid block for rendered d2 svg

* ci(docs): install d2 before building the site

* docs(architecture): tighten layout and reroute label-crossing edges

Flip device/sidecar order so trace writer drops cleanly to runs/
without cutting through the JVM cell, right-align the inspect row
via a pad column, and tune grid gaps to keep gRPC and Unix socket
labels off the SANDERLING boundary.

docs/_template/page.html

@@ -4,6 +4,9 @@
 <meta charset="utf-8">
 <meta name="viewport" content="width=device-width,initial-scale=1">
 <title>$if(title)$$title$ · $endif$sanderling</title>
+<link rel="preconnect" href="https://fonts.googleapis.com">
+<link rel="preconnect" href="https://fonts.gstatic.com" crossorigin>
+<link rel="stylesheet" href="https://fonts.googleapis.com/css2?family=Inter:wght@400;500;600;700&family=JetBrains+Mono:wght@400;500;700&display=swap">
 <link rel="stylesheet" href="__ROOT___assets/style.css">
 <style>$highlighting-css$</style>
 </head>
@@ -17,6 +20,7 @@
         <li><a href="__ROOT__manual/getting-started.html">Getting started</a></li>
         <li><a href="__ROOT__manual/writing-specs.html">Writing specs</a></li>
         <li><a href="__ROOT__manual/runs.html">Runs</a></li>
+        <li><a href="__ROOT__manual/inspect.html">Inspect</a></li>
         <li><a href="__ROOT__manual/cli.html">CLI reference</a></li>
       </ul>
       <h3>Development</h3>