… Principle VIII.

Adds smoke coverage for the v2 surface area introduced in #3176 that the
existing flat-token tests didn't reach.  All additions land in
tests/theme_manager_test.cpp — the CMake target already links every TU
the new assertions touch (ThemeManager.cpp, AppSettings.cpp,
LogManager.cpp, AsyncLogWriter.cpp), so no CMakeLists change is needed.

New blocks:

JSON loader/writer
- v2 primitives + {alias} resolution end-to-end through color() /
  cssFragment() / resolve() — guards against the cssFragment-empty-for-
  ThemeFont class of bug that surfaced mid-PR.
- flattenTokens discriminator routing: a single v1 file with a gradient
  ({"type":...}), a compound font ({"family":...}), and a plain nested
  group is imported and each leaf is type-checked.  Pins the
  discriminator order in flattenTokens().
- v1 → v2 migration round-trip: import v1 file, mutate via setColor
  (triggers auto-save), re-read on-disk JSON to assert schemaVersion=2 +
  scopes.root.tokens wrapper, then reload and confirm values survive.

Scope tree (pure in-memory on the built-in Default Dark; saveActiveTheme
silently fails for built-ins so each block leaves no disk side-effects)
- setColor at a nested scope does not touch root.
- Inheritance walk: token defined at scopeB read from scopeB/leaf
  resolves to scopeB's value.
- removeOverride at a nested scope drops the override; reads fall back
  to parent.
- removeOverride at root is the defensive no-op (would otherwise delete
  the token tree-wide).
- scopeOrCreate("a/b/c") wires up the full chain, verified via
  containerPaths() + isOverriddenAt() on each intermediate.
- Each scope-tree block uses a unique container path (scopeA / scopeB /
  scopeC / deep) so state from earlier blocks can't bleed into later
  assertions.

Compound font tokens
- setFontToken → fontToken round-trip preserves all three fields.
- value() on a ThemeFont downgrades to .family (the ~35 legacy callers).
- cssFragment("font.size.freq") + sizing("font.size.freq") virtual
  lookups return the compound's embedded size — the bug fixed mid-PR.
- Persistence: the v1→v2 migration block doubles as the compound-on-disk
  check (asserts {family, size, color} JSON shape, not a nested scope).

Editor-surface smoke (deferred-commit + per-theme recent-colors) is
deferred to a follow-up that can stand up the GUI dialog layer in its
own test target.

Blast radius: skipped — additive test-only change, no production
symbols touched.