Add Entity<TModel>(repository, string tableName) constructor overload

[bart-bot-boy]

Summary

Adds a further-simplified constructor overload on Entity<TModel> (and the base Entity<TModel, TFilter, TOptions> / Entity<T, TOptions> / Entity<TModel, TUpdate, TFilter, TOptions> constructor chain) that accepts a plain string tableName, e.g.:

var entity = new Entity<Identity>(repository, "I");

This is equivalent to today's:

var entity = new Entity<Identity>(repository, new Table<Identity>("I"));

Internally the new overload just delegates to the existing (repository, Table<TModel>) constructor, constructing the Table<TModel> from the given name so callers don't need to know about Table<T> for the common case. The existing (repository, Table<TModel>) constructor is untouched and remains available for advanced cases (custom schema, quoting, reusing an existing Table<T>, etc.).

Docs (docs/USAGE.md) were updated to show the simplest string-based form first, noting the Table<T> overload as the more advanced option, per the reasoning from the linked review discussion.

Changes

• src/Simpleverse.Repository.Db/Entity/Entity.cs: added Entity(DbRepository repository, string tableName) overloads on Entity<TModel, TUpdate, TFilter, TOptions>, Entity<TModel, TFilter, TOptions>, Entity<T, TOptions>, and Entity<T>, each delegating to the existing Table<TModel>-based constructor.
• src/Simpleverse.Repository.Db.Test/SqlServer/Entity/EntityTest.cs: added a new IdentityEntityByTableName test helper class built with the new string-based constructor, plus a test (Constructor_WhenGivenTableNameString_CreatesEquivalentTableToTableConstructorOverload) asserting it produces an equivalent Table<T>/table name to the existing Table<T>-based constructor.
• docs/USAGE.md: reordered the "Defining an Entity" quick-start section to lead with the new string-based constructor, with the Table<T> overload described afterwards as the more advanced form.

Test plan

• dotnet build src/Simpleverse.Repository.Db/Simpleverse.Repository.Db.csproj – succeeds, 0 warnings/errors.
• dotnet build src/Simpleverse.Repository.Db.Test/Simpleverse.Repository.Db.Test.csproj – succeeds (only pre-existing xUnit analyzer warnings, unrelated to this change).
• Could not execute the new/existing SQL Server-backed test suite in this sandbox (no SQL Server instance available, and this project targets net6.0 test host while only .NET 8 SDK/runtime could be installed here due to environment constraints). The new test compiles cleanly against the same DatabaseTestFixture/SqlRepository pattern used by all other tests in EntityTest.cs, and only exercises the pure C# constructor delegation (no actual DB round-trip), so it should behave identically to the other entity-construction tests in this file once run against a real SQL Server, as CI presumably does.

Risks / things to double check

• I did not modify ProjectedEntity<...> constructors — they take an IEntity<...> instance rather than a Table<T>/table name, so the same simplification doesn't apply there. Worth confirming that's the intended scope per the issue.
• Please verify the new overloads don't introduce ambiguity with any existing implicit conversions from string to Table<T> elsewhere in the codebase (none found in review, but worth a second look).
• The new test only asserts TableName equivalence (via a small public TableName property added to the two test helper entity classes) since running the DB-backed test suite wasn't possible in this sandbox; a maintainer with access to CI/a SQL Server should confirm the new test passes end-to-end.

Closes #24