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/_assets/style.css

@@ -6,6 +6,8 @@
   --accent: #0a66c2;
   --code-bg: #f6f6f6;
   --sidebar-width: 240px;
+  --font-sans: "Inter", -apple-system, BlinkMacSystemFont, "Segoe UI", system-ui, sans-serif;
+  --font-mono: "JetBrains Mono", ui-monospace, SFMono-Regular, Menlo, Consolas, monospace;
 }
 
 @media (prefers-color-scheme: dark) {
@@ -25,9 +27,9 @@ html { background: var(--bg); color: var(--fg); }
 
 body {
   margin: 0;
-  font-family: -apple-system, BlinkMacSystemFont, "Segoe UI", system-ui, sans-serif;
-  font-size: 16px;
-  line-height: 1.55;
+  font-family: var(--font-mono);
+  font-size: 14px;
+  line-height: 1.65;
 }
 
 .layout {
@@ -49,20 +51,24 @@ body {
 
 .sidebar .brand {
   display: block;
+  font-family: var(--font-mono);
   font-weight: 700;
-  font-size: 1.25rem;
+  font-size: 1.1rem;
+  text-transform: uppercase;
+  letter-spacing: 0.08em;
   text-decoration: none;
   color: var(--fg);
   margin-bottom: 1.5rem;
 }
 
 .sidebar h3 {
-  font-size: 0.75rem;
+  font-family: var(--font-mono);
+  font-size: 0.7rem;
   text-transform: uppercase;
-  letter-spacing: 0.05em;
+  letter-spacing: 0.08em;
   color: var(--muted);
   margin: 1.25rem 0 0.25rem;
-  font-weight: 600;
+  font-weight: 500;
 }
 
 .sidebar nav ul {
@@ -89,15 +95,32 @@ main {
   width: 100%;
 }
 
-main article h1 { font-size: 2rem; margin: 0 0 1.5rem; line-height: 1.2; }
-main article h2 { margin-top: 2.5rem; font-size: 1.4rem; }
-main article h3 { margin-top: 2rem; font-size: 1.1rem; }
+main article h1,
+main article h2,
+main article h3,
+main article h4 {
+  font-family: var(--font-mono);
+  font-weight: 700;
+  letter-spacing: -0.01em;
+}
+
+main article h1 { font-size: 1.9rem; margin: 0 0 1.5rem; line-height: 1.2; }
+main article h2 { margin-top: 2.5rem; font-size: 1.3rem; }
+main article h3 { margin-top: 2rem; font-size: 1.05rem; }
+
+main article img {
+  max-width: 100%;
+  height: auto;
+  display: block;
+  margin: 1.25rem 0;
+  border-radius: 4px;
+}
 
 a { color: var(--accent); }
 
 code {
-  font-family: ui-monospace, SFMono-Regular, Menlo, Consolas, monospace;
-  font-size: 0.9em;
+  font-family: var(--font-mono);
+  font-size: 0.88em;
   background: var(--code-bg);
   padding: 0.1rem 0.3rem;
   border-radius: 3px;
@@ -108,7 +131,8 @@ pre {
   padding: 1rem;
   border-radius: 4px;
   overflow-x: auto;
-  font-size: 0.875rem;
+  font-family: var(--font-mono);
+  font-size: 0.85rem;
   line-height: 1.5;
 }