Initial boilerplate scaffold for continuum-common

Adds ts-core (logger, Supabase JWT verification, Elysia error
transformers) and rust-core (thiserror error type, tracing init,
G-code checksum/header parsing) packages.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

.env.example

@@ -0,0 +1,6 @@
+# continuum-common is a library repo (ts-core / rust-core) and has no runtime
+# of its own. These vars are only needed when running this package's tests,
+# which exercise the Supabase JWT verification helper against real settings.
+
+SUPABASE_URL=https://your-project.supabase.co
+SUPABASE_JWT_SECRET=your-supabase-jwt-secret