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.

Makefile

@@ -14,6 +14,8 @@ SANDERLING_BIN := bin/sanderling
 DOCS_SRC := $(shell find docs -type f -name '*.md' -not -path 'docs/_*')
 DOCS_OUT := $(patsubst docs/%.md,build/site/%.html,$(DOCS_SRC))
 DOCS_TEMPLATE := docs/_template/page.html
+DIAGRAM_SRC := $(shell find docs/_diagrams -type f -name '*.d2' 2>/dev/null)
+DIAGRAM_OUT := $(patsubst docs/_diagrams/%.d2,build/site/_assets/diagrams/%.svg,$(DIAGRAM_SRC))
 
 INSPECT_DIST := internal/inspect/dist
 WEB_DIST := web/dist
@@ -82,14 +84,18 @@ test-kotlin:
 test-spec-api:
 	cd pkg/spec-api && npm test --silent
 
-docs: $(DOCS_OUT) build/site/_assets
-	@echo "built $(words $(DOCS_OUT)) pages to build/site"
+docs: $(DOCS_OUT) build/site/_assets $(DIAGRAM_OUT)
+	@echo "built $(words $(DOCS_OUT)) pages, $(words $(DIAGRAM_OUT)) diagrams to build/site"
 
 build/site/_assets: docs/_assets
 	@mkdir -p build/site
 	@rm -rf $@
 	@cp -R $< $@
 
+build/site/_assets/diagrams/%.svg: docs/_diagrams/%.d2
+	@mkdir -p $(dir $@)
+	@d2 --theme 301 --pad 20 $< $@
+
 build/site/%.html: docs/%.md $(DOCS_TEMPLATE)
 	@mkdir -p $(dir $@)
 	@pandoc $< --from=gfm --to=html5 --standalone \