Design decisions

The choices that shape everything else in the engine, recorded in a compact decision-record format: the context at the time, the decision, and the consequences that followed - including the trade-offs accepted, which are as much a part of the record as the benefits.

These records are point-in-time. Each states what was decided and why; the internals references describe how the resulting subsystems work today, and take precedence wherever detail has evolved.

#	Decision
001	Arrow is the data plane, not an integration
002	Embedded-first: the engine is a library before it is a cluster
003	State is an open dataset, not an internal format
004	Jobs deploy as compiled plugins
005	The SQL optimizer is hand-rolled, not adopted
006	Incidents replay deterministically from captured input