feat: DomainModeling.V2 — simplified session API for UI/API/MCP consumers

[Copilot]

Introduces a new Poly.DomainModeling.V2 namespace with a streamlined, exception-safe authoring surface designed for UI, API, and MCP consumers. The core design principle: every mutation returns a DomainTransactionResult — no exceptions escape the session boundary.

New surface (Poly/DomainModeling/V2/)

• DomainSession — wraps Domain with revision tracking (64-snapshot rolling window), cached AnalysisResult, and atomic Apply(intent[s]) that catches both intent-dispatch exceptions and analysis rollbacks into a uniform result
• DomainSessionStore — thread-safe registry; Create() bootstraps canonical built-in types and seeds revision 0
• DomainDispatcher — named convenience wrappers over session.Apply() (AddEntity, AddRelationship, etc.)
• DomainSessionIntentExtensions — same surface as fluent extension methods on DomainSession
• DomainModelSnapshot + SnapshotBuilder — immutable record projection of the live domain for serialization/diff
• DomainRenderer — ASCII rendering for full domain, single entity, and stage summaries
• ECommerceDemo — full e-commerce domain (Customer/Order/Product/Shipment/Payment/Review/Category) ported to V2 intent-based authoring with low-level effect/policy wiring
• DomainSessionBuilder (internal) — fluent test builder for isolated unit tests

Engine change

DomainMutationIntentEngine gains ApplyWithTrace() overloads and an internal Populate() method, factoring the existing Apply() through a shared PopulateAndApply() path without behavioral change.

Usage

var store = new DomainSessionStore();
var (sessionId, session) = store.Create("Order Management");

// High-level extension methods
session.AddEntity("Order");
session.AddProperty("Order", "Status", "Text");  // separate Apply — intent dispatch resolves against committed state
session.AddStage("Order", "Pending");

// Dispatcher style
var dispatcher = new DomainDispatcher(session);
dispatcher.AddRelationship("CustomerOrders", "Customer", "Order", RelationshipCardinality.OneToMany, sourceOwnsTarget: true);

// Always returns a result, never throws
DomainTransactionResult result = session.Apply(new AddActionToEntityIntent("Order", "Cancel"));
// result.Succeeded, result.Revision, result.Diagnostics, result.Trace

// Immutable snapshot for serialization
DomainModelSnapshot snapshot = session.ToSnapshot();
string report = session.RenderAsText();

Key constraint: intent dispatch resolves object references against the already-committed domain state, so entity/event-type creation and subsequent property/stage additions must be in separate Apply() calls.

Original prompt

Create a new simplified DomainModeling.V2 implementation from scratch optimized for UI, API, and MCP consumers. Include: immutable core records, unified mutation dispatcher, session management with transactions/traces, high-level intent tools, e-commerce demo ported, analysis and rendering preserved. Place in Poly.DomainModeling.V2 namespace/folder. Make internal builder for tests.