Reorg introduce api package containing Entry interface

[steiler]

reorganize the tree package by introducing a new pkg/tree/api surface and a pkg/tree/consts package, then updates the codebase to use those exported interfaces, types, and constants.

• Adds a dedicated API layer (pkg/tree/api) with Entry, LeafVariants, delete-path types, and non-revertive tracking; introduces pkg/tree/ops helpers (e.g., GetByOwner) and an ops interface.
• Moves tree constants to pkg/tree/consts and replaces many direct tree.RunningIntentName/RunningValuesPrio/Replace* usages with consts equivalents across datastore, targets, server, and tests.
• Refactors core tree implementation to use the new API types: method visibility changes (e.g., ShouldDelete, GetOrCreateChilds, ToJsonInternal), LeafVariants accessors, and TreeContext now exposing SchemaClient, ExplicitDeletes, and NonRevertiveInfo.
• Updates mocks and generated code to reflect new API package paths and added interface methods; adds a new mock for SdcpbPath.
• Adjusts update/path handling with SdcpbPath() and updates callers, plus broad test updates to match new APIs and constants.