0643 — One declaration per type, vulkan as the client plus aliases

Context. [0625] said “every user-spelled type lives in vulkan” and the ROADMAP carried the follow-through: move the declarations in. Building it is impossible in Go: a package that declares TopicConfig and is imported by the topic controller cannot also import the assemblers that read it — the import graph forbids one package both declaring the types and assembling the machinery. Alias chains ran four hops (vulkan -> producer -> controller -> datastore), the hand-kept alias list was missing a dozen names, errors and events were never re-exported, and seven codes sat below a root under unexported names. River’s facade-plus-types shape, not Temporal’s alias-into-internal, is the shape.

Decision. Four laws. (1) One declaration: every exported type, const, named error, and declared event is declared once; the only type X = pkg.X lines in the repo are pkg/vulkan/alias.go. (2) Placement by lowest reader, with a floor: machinery (controller, datastore, batcher, worker) declares nothing a user spells except its own Config and *Row structs and its controller / datastore / instance / provisioner types, so a user-spelled name lives in exactly one of common (two domains read it), a root (that domain’s machinery reads it), or an assembler (only its own verbs read it). Codes are the same law with no assembler case: every NewDiagnosticError / NewDiagnosticEvent / NewDiagnosticMetric initializes an exported var in a root’s errors.go, events.go, or metrics.go, or in common. (3) Roots are named for what they are about: thing domains take the resource (topic, system, worker, alert, metrics), activity domains the verb (schedule, migrate, compaction, consume, produce), an activity’s assembler its agent noun (scheduler, consumer, producer); a root’s own controller is <Root>Controller. A root type whose bare name is a generic noun takes the root’s noun as prefix at the declaration (MetricKind, AlertStatus, DiagnosticError; consts follow), so an alias stays same-named. (4) vulkan is the client plus aliases: Client, ClientConfig, the pool, four handles, two instance wrappers, two interfaces; everything else an alias or var into the declaring package, the set computed by a go/types closure test; the client holds assemblers only.

Amends [0625]: “every user-spelled type lives in vulkan” becomes “aliased in vulkan from its declaring package”; the rest stands. Amends [0555]: the domain kind gains the naming rule, the one-declaration law, and the floor; consumergroup is consume, produce is a root, assemblers declare no codes.

Consequences. consumergroup -> consume, producer machinery -> produce, Versioned -> common, Tx/InTransaction -> datastore, TopicConfig / ScheduleConfig / ScheduleSpec / SystemConfig / alert JobConfigs to their roots; ConsumerConfig gains Bindings; admin gains GetGroup / ListGroups / ListGroupWorkers / GetCompactionHead / ListKeyMessages so a handle verb is one call; vulkan’s adapter.go and config twins are deleted. Six tools/conventions tests enforce the laws: alias placement, vulkan’s imports, the closure comparing targets, the machinery floor, codes at roots (doubling as registry completeness), SQL owner segments. Rejected: an internal/ tree (not now); a pkg/message root; dissolving admin into vulkan; dropping type prefixes (a separate pass); a hand-kept alias list.