feat(mvux): advanced mocking & previews — spec 013 + implementation (WIP) - #3165

- spec.md: audience/deliverable split, derived-feeds-survive flow,
  three-tier layering, generator responsibility split, end-to-end
  sequence, tier-1 evolution sequence; risks and decision log as tables;
  status header cleaned for team/CEO review (history stays in history.md)
- architecture.md: swap-anchor mermaid in section 1, end-to-end ASCII
  flow replaced by a sequence diagram in section 5

…he swap fail hard

Records the decision to drop the dedicated static flag in favour of a per-context bit, and
to reuse the hot-reload reflection driver with strict failure instead of best effort.

Adds a per-context bit inherited at creation and reads it where the wrap is decided, so only
contexts created under an activation scope wrap their feeds and a live application pays
nothing.

Classifies every feed member as service dependent, derived or independent, and instruments
the constructors so an eagerly dereferenced service is reported. The metadata is what the
consumer generator reads.

Commands have no hot-swap state, so a dedicated seam reassigns the command property after
construction. Construction itself needs no seam: the public constructors plus the ambient
scope are enough.

Adds the activation scope, the swap engine and the typed feed vocabulary used by the
generated mocks.

The consumer generator reads the app metadata and emits the mock record, the Create
factories and the SetModel facade.

StateImpl always implements IHotSwapState, so the previous type test never rejected a
non-wrapped feed and the swap silently did nothing. Added CanHotSwap and made the engine
throw when a mocked feed is not swappable.

Adds Value, Empty, Undefined, Loading, Error and Refreshing for both scalar feeds and list
feeds, built on pinned messages and strongly typed end to end.

…cabulary

The generated record exposes Empty with every input pinned to its empty state, Create()
builds from it, and commands can be overridden through the view-model seam.

Logs what landed for the tier 2 and 3 implementation and what is left out of scope.

The mocked feed vocabulary already exists in Uno.HotTesting.Reactive, so the duplicate
assembly is removed and everything lives there: FeedMock and ListFeedMock are reused,
CommandMock is added, and the swap engine moves into MockingService as strongly typed
helpers. The consumer generator becomes Uno.HotTesting.Reactive.Generator and emits raw
string literals.

The MVUX instrumentation is now emitted by default, with an assembly level opt-out.

…ourceContext

The ambient activation state is a mocking concern, not a Core one. MockingService now owns
the AsyncLocal and registers a probe; SourceContext only keeps the per-instance
IsMockingActive bit. In a live application the probe is never registered, so the bit stays
false and the cost is zero.

… Create

Review feedback: the generated {Vm}Mock is now partial so applications can extend it with
named catalogs, SetModel is renamed to SetMock, Create only takes the mock record,
{Model}Mock.Empty stays on the record so it composes with `with`, and Create opens the
MockingService.Enable() scope itself so user code never has to.

Command mocking is deferred to a future version: the consumer generator no longer emits a
command member nor the __Mock_SetCommand wiring.

Applied: combined the nested if in ViewModelGenTool_3.Mocking, projected
ctor.DeclaringSyntaxReferences, SimpleNameSyntax and the accesses with Select, and
filtered the nested types and the references explicitly in FeedsMockGenerator.

Declined, each answered in its thread: the readonly suggestion on Disposable._onDispose
is a false positive (the field is a ref argument of Interlocked.Exchange), the == false
simplification targets a bool? whose three-state semantics are intentional, and the Where
suggestion filters a computed value.

Everything pushed to GitHub must be in English. The spec 013 history document was
written in French during the design discussions, which failed the CI spell-checking
validation (333 unknown words). spec.md, architecture.md and the reference doc were
already clean.

- history.md fully translated to English, keeping every SHA, type name, file path and
  decision id unchanged.
- implementation.md: replaced three coined words the dictionary rejects (bikesheddable,
  Authorable, mockability) with plain English equivalents.

Verified with the exact CI commands: cspell over 236 markdown files reports 0 issue, and
markdownlint reports no violation.