Add AvailableEnvVars under StaticFiles:ParseContentOptions. Listed env
vars are resolved once at parser construction, JSON-escaped (like claims)
and templated into static content via the existing {NAME} tag machinery.
Aimed at SPAs on K8s: inject per-environment values (build label, feature
flags, analytics IDs) from pod env vars without rebuilding the bundle.

- Both AvailableEnvVars and AvailableClaims now accept either an array of
  names or an object of name->default pairs. Missing env var falls back to
  its configured default, then the empty string; missing claim falls back
  to its default, then NULL (historical behaviour).
- Values resolved once at startup (pod restart re-reads); claims win over
  env vars on name collision.
- Allowlist-only: only listed names are read, never the whole environment.
  Distinct from the server-side Config:ParseEnvironmentVariables path.

New Config.GetConfigNameDefaults reader handles both forms. Registered the
key in ConfigDefaults, ConfigTemplate, ConfigSchemaGenerator and
appsettings.json. Tests: 10 parser unit tests + 2 config-validation tests.

The design doc is no longer needed now that the feature has shipped in
this branch; keep it out of the repo tree.