Settings snapshot schema 2 sends each design once

Status: accepted. Settings contract 0.3.0 (additive), snapshot schema 2 beside schema 1. Follows plain followers read a summary.

Context

A snapshot carries one compiled design projection per effective context: the desktop and each native app. With the embedded design every context shares one projection, so a 499 KB schema 1 snapshot holds seven copies of one 71 KB projection. Native consumers (the portal, the compositor's scene host and applications on the shared settings consumer) need those projections, receive the snapshot as a retained topic delivery and read it again to confirm, so each of them moves about 1 MB per change.

Several of those consumers belong to applications still awaiting their port to this tree's egui toolkit. They carry their own copy of the settings library and must keep working, unchanged, until each is ported.

Decision

1. Schema 2 is a wire form. settings::compact::CompactSnapshot holds each distinct projection once under designs, keyed by its digest; each context's design is a digest naming one. compact::decode expands schema 1 or 2 into the same in-memory Snapshot, so the consumer, reducer, cache and effective digests do not change. A context naming an unknown design, or a design no context names, is refused. Digests are opaque keys to the decoder.
2. Asked for, never assumed. settings.get (full view) and settings.appearance.get take schema: 1 is the default and is never sent; 2 answers in the compact form; any other is refused with unsupported_schema.
3. A compact topic. settingsd publishes the retained snapshot on three topics, in order: settingsd.desktop.changed.<profile> (schema 1), settingsd.desktop.compact.<profile> (schema 2), then the summary. A revision is published once all three are; a failure retries all three. noded reserves the compact topic for settingsd as it does the others.
4. Ported consumers move now. The shared native consumer (settings::native) follows the compact topic and reads schema 2, and the portal asks for schema 2. Measured with the embedded design, the compact snapshot is about 72 KB where schema 1 is 499 KB.
5. Unported consumers move with their port. An application or service still on its own copy of the settings library keeps following the schema 1 topic and reading schema 1. Each moves when it is ported, by using the shared consumer. Nothing in an unported consumer changes for this decision.

Retiring schema 1

The schema 1 topic and default stay while any consumer still uses them. Publishing schema 1 costs a full snapshot per change, so once the last consumer has moved, a later decision stops publishing it and makes schema 2 the default read. The library keeps decoding schema 1 for caches and old retained state.

Not chosen

Deltas between revisions, compression and local compilation are discussed in the summary decision. Schema 2 removes duplication without changing what a consumer holds; local compilation (no projection on the wire) remains the next step once consumers can pin the design source by digest.