feat(foundation): backend seam (reads+writes), typed models, four-state mock, macOS CI, CDC tracer

[laksamanakeris]

Squash of the 16-task foundation milestone. Atomic 16-commit TDD history preserved on feat/foundation. 26 files, +1267/−583.

Why

The app runs on mock data behind a single ConnectionService trait. Before real NodeDB connectivity lands (blocked on NodeDB 0.5), the seam needed to be shaped so the real client plugs in without reshaping every screen: per-topic traits, typed models instead of tuples, a mock that can simulate all four async UI states, and a CI gate so regressions can't land silently. This PR is that foundation.

What changed

Backend seam — one front door, per-topic traits

• services/backend.rs (new): Backend supertrait composing ConnectionService + StreamsData, with a blanket impl. The app provides ONE Rc<dyn Backend>; each screen reads only the method it needs. Adding a future domain trait (Explorer, Admin, …) extends the bound — additive, never a reshape.
• services/streams_data.rs (new): Streams-tier reads. One method per screen's data; today cdc_feed(). The seam — not the view — owns the native-doc → display-JSON mapping (sonic_rs at the boundary).
• services/connection_service.rs: first write through the seam — mark_all_read() with persistent mock state, so the notification feed stays consistent after reload (fixes POP-03: unread badge reappearing after mark-all-read).

Four-state mock

MockConnectionService gains ready() / empty() / erroring() constructors plus a delayed mode (MockBehavior), so every async screen can be driven through loading → loaded / empty / error in tests and demos without a server.

Typed CDC models

• models/cdc.rs (new): CdcRow { id, time, op, collection, payload_json } + CdcOp enum replace the anonymous tuples the view carried. Stable id enables keyed Dioxus lists (no index keys).

Streams · CDC through the seam

• views/streams/cdc.rs: rewritten to fetch via cdc_feed() with a fetch/presentational split; simulated live stream driven by tokio time with a working Pause; four-state SSR render tests.
• components/live_tail.rs (new): reusable LiveTail layout shell (toolbar/body/footer slots) so future live screens (Notify, MV, Console) compose the same chrome instead of re-emitting it.
• components/sparkline.rs (new): pure-Dioxus SVG sparkline spike — proves chart fidelity without a JS charting dep.

Mock data reorganization

data/mock.rs (523 lines, over the 500-line repo limit) split per-domain into data/mock/{connections,cdc,docs,notify}.rs.

CI (first remote run = this PR)

.github/workflows/ci.yml (new): macOS runner, cargo fmt --check + clippy -D warnings + cargo nextest run --workspace, with cargo caching. This PR is the workflow's first-ever execution on GitHub.

Tests

60/60 passing locally (fmt + clippy clean). Coverage added by this PR includes: mark-all-read persistence across reads, four-state cdc_feed behavior, loading→loaded transition without holding a signal guard across .await, stable/unique row ids, keyed-row SSR rendering, LiveTail slot rendering, sparkline SSR output.

Deferred (tracked, intentionally out of scope)

• notify.rs / explorer/sidebar.rs still bypass the seam.
• CDC live-stream vs base-state ownership question (design note recorded).

Merge note

Rebase / fast-forward merge to keep main at exactly one commit for this milestone; GitHub's squash-merge would rewrite the hash and message.