Resolve key fields declared in nested messages (for example metadata.id) across key lookup, indexability checks, order planning, and field-reader path resolution. Add recursive protoc-gen validation to reject invalid/ambiguous nested key declarations and extend tests for nested-key behavior.

Signed-off-by: Adphi <philippe.adrien.nousse@gmail.com>

…stry cache

Move key field/value resolution and data-prefix derivation behind registry APIs so nested key paths are resolved consistently across db/index paths.
Keep legacy inferred-key precedence and add targeted cache invalidation to avoid behavior drift while reducing repeated key lookup logic.

Signed-off-by: Adphi <philippe.adrien.nousse@gmail.com>