stack-encrypt refactor: remove transitional traits and redundant execution APIs #1488

target::legacy is migration scaffolding. Finish the migration before declaring the stable public API complete.

Acceptance:

• Inventory every consumer of legacy borrowed conversions and old EncryptFrom/DecryptInto declarations.
• Migrate derives, plans, targets, EQL, guest/binding code, examples and generic callers to the owned declaration/conversion surface.
• Remove replaced traits/helpers and redundant public encrypt_as-style conveniences once their workflows have equivalents.
• Keep explicitly supported encrypt_tree/decrypt_tree semantics distinct from ordinary opaque encryption.
• Publish one coherent import/signature/receiver map and make all representative scalar/blob/record/searchable/JSON examples compile.
• Pass compile-fail, batching, lifecycle and cross-language conformance checks; preserve released SDK and storage contracts.

Depends on #1483, #1486, #1484 and #1485 as applicable. Do not add compatibility aliases solely for the unpublished prototype; account for any customer contract established before removal.

Plan: conformance roadmap and API redesign #1365.