Engine architecture

Layers

Library	Responsibility
`xbase`	Table runtime, work-area objects, records, cursor state, field metadata, table flavors
`xindex`	Index containers, tag and order behavior, backend index work
`memo`	Memo references, managers, objects, stores, verification
`xexpr`	Expression evaluation
`value`	The shell's value system
`tui`	Full-screen text interface

These are not conceptual layers. Each is a separate link target, and the boundaries between them are checkable in a single build log. Source-evidenced

The execution chain

main → shell → command registry → one translation unit per command → libraries

The front end owns nothing

Ordering, cursor state, relations, validation, and command execution live in the engine and the shell. Terminal, text-mode, and desktop surfaces are consumers of that truth, never duplicators of it. A front end can be added or removed without the database behaving differently. Source-evidenced

The working rule

Conventions suggest.
Registration declares.
Metadata records.
Runtime proves.
Validators enforce.