docs(adr): ADR 0024 — list + live-preview workspace concrete design

[jinyeow]

Closes #42. Process gate 1 of the flagship (tracking issue #36): merging this PR = design sign-off — no Track 2 build ticket starts before it lands.

Records the decided (2026-07-20, A/A/A) concrete design for ADR 0022 stage B as ADR 0024, condensing docs/flagship-preview-design.md:

• Always-on preview — preview = auto|off in config.toml, auto-collapse thresholds (prototype-tunable), collapsed = today's UX.
• Two-tier load — tier 1 instant from list-row data; tier 2 on ~200 ms debounced cursor-settle via injected TimeProvider.
• Tab claims pane focus in the workspace — deliberate break of the global Tab=NextTab alias there; [/] stay canonical (ADR 0021); ADR 0023 remap is the mitigation.
• The load invariant, verbatim as spec (atomic PreviewState publish, keyed-on-displayed-item + seq stamp, single-flight cancel-superseded with faults observed, debounce) — reject-on-violation, typed via the flagship foundation: H3 — SingleFlightCache<K,V> + Published<T> (typed ADR-0008 invariants) #46 foundations.
• The reuse boundary — H2 pure formatters shared by dialog + preview (width + tier params, subset-not-clone), all actions and the whole DiffReviewDialog stay modal, detail VMs per previewed item.

Also: ADR 0022 stage-B entry links here; ADR README indexed.

All code references verified against current main (DiffState PrDiffViewModel.cs:81, _loadSeq in both list VMs, FocusLeft/Right + Tab bindings KeyBindingTable.cs:49/114-115, ResponsiveLayout.Compute).