0007. Reserve append / CAS / watch primitives

Context

A Drive-like product reads and writes whole files. A future collaborative editor streams small operations against shared documents and needs change notification. Building the collaboration layer is out of scope, but three storage primitives decide whether one can be built on top, and they are cheap to design in now and expensive to retrofit.

Decision

Reserve three primitives in the commit protocol and metadata schema from the start, as non-goals-with-reserved-seats (not built early, but accommodated):

1. Atomic append — commit "extend file with this data iff current length is L" (a conditional mutation the commit protocol already supports).
2. Compare-and-set — commit iff version matches (the meta:version counter already present for optimistic concurrency).
3. Watch / notify — "tell me when anything under this directory changes," as a metadata-layer event stream. The hook must exist in v1; notification fan-out at scale can come later.

Consequences

• The collaborative-editor future is buildable without re-architecting storage.
• Watch also serves Drive sync clients, so it earns its place even before any editor exists.
• The commit protocol and schema must accommodate these from Milestone 0 (section 9), even though the features ship later.