Initial CAC3 explorer — design doc, license, config schema.

This is a docs-only seed. Tomorrow's session will add the actual Go
source: main.go, internal/rpc/client.go (JSON-RPC + cookie auth),
internal/chain/, internal/explorer/ (handlers), and internal/view/
(templates + static assets, embed.FS).

Until then this commit gives the repo its shape: README with the full
design, config example with all knobs, MIT license to match the
chain, and a .gitignore for Go build artifacts.

.gitignore

@@ -0,0 +1,31 @@
+# Go
+/bin/
+/dist/
+*.exe
+*.test
+*.out
+coverage.txt
+*.prof
+
+# Local config / secrets
+.env
+config.local.yaml
+*.cookie
+
+# Editor / OS
+.DS_Store
+.idea/
+.vscode/
+*.swp
+*~
+
+# Build cache
+/tmp/
+/cac3-explorer
+/cac3-explorer-*
+
+# Go modules (commit if not vendored; comment out if vendoring)
+# vendor/
+
+# Tailwind / static build cache
+/static/css/.tailwind-cache/