Issue #3607693: Chain [orchestra:var:NAME] into a referenced entity's tokens

A process variable often holds an entity reference id (a captured completer uid, a node id). Showing a field of the referenced entity previously meant snapshotting each field into its own variable, which does not scale.

Let a var: token name a target entity type and chain into that entity tokens: [orchestra:var:validator_uid:user:display-name], [orchestra:var:some_node:node:title]. ProcessTokens loads the entity by the variable value and defers the rest to the core token generator for that type, reusing every [user:*]/[node:*] token. The bare [orchestra:var:NAME] still returns the raw value; the type is explicit (never guessed); an unknown type or unloadable reference clears. Kernel test covers user display-name/mail, the bare uid, an unknown type and a missing reference (10 tests). Follow-up to #3607623.