0563 — Coded declarations move to pkg/common/diagnostic

Context

After [0561]/[0562], common root still held the error anatomy, the log events, and their shared registry — three files of one system mixed into the vocabulary grab-bag, with LogEvent the odd name out beside Error. [0561]‘s “errors stay flat” rested on a common/errors package shadowing stdlib errors; a package named for the concept dodges that entirely.

Decision

• pkg/common/diagnostic (package diagnostic) owns the coded-declaration system: registry.go (Declaration, Kind = KindError | KindEvent, the one map, register, listRegistered[D], isVKCode), error.go (Error, NewError, Recovery, renderers’ parts, Errors()), event.go (Event, NewEvent, Events()).
• The name: an error and an operator-actionable log event are both diagnostics — coded, explainable, docs-paged. vulkan explain explains a diagnostic.
• LogEvent renames to Event across the system: NewEvent, Events(), KindEvent — the package qualifier carries the “log” half (diagnostic.Event).
• Domain declarations now read diagnostic.NewError / diagnostic.NewEvent; errors.go / logs.go file split in domains unchanged.
• common root keeps its own four Err* declarations (errors.go) and the retry classification (IsTransientDatastoreError branches on diagnostic.Error); imports point strictly downward — diagnostic imports no vulkan package.

Consequences

• Narrows [0561] again: common root is now vocabulary only; the two subpackages are diagnostic (coded declarations) and logging (machinery).
• ~100 qualifier renames across 18 files + CLI + tools walks (completeness regex now New(?:Error|Event)).