0.10: Add application testing builders, fixtures, and unit-of-work fakes

[bazer]

Objective

Provide first-class database-free application testing support that preserves DataLinq metadata, key, relation, Memory, and unit-of-work contracts instead of encouraging invalid runtime mocks or pretending in-memory execution proves SQL-provider behavior.

Required scope

W6A: builders, relations, and Memory fixtures

• Metadata-aware immutable builders with valid row, table, primary-key, equality, GetValues(), and Mutate() behavior.
• Collection and reference relation doubles implementing their full supported interfaces.
• Relation graph builders driven by DataLinq relation metadata.
• Deterministic fixture construction, seeding, reset, IDs, clocks, and defaults where owned by this testing surface.
• Registration over the real DataLinq.Memory capability set, preserving unsupported-query failures.

W6B: fake unit of work and DI replacement

• Fake unit-of-work behavior derived from the production H10 contract, including write recording, commit/rollback/disposal, failure injection, and terminal states.
• DI replacement helpers with unambiguous names for Memory-backed tests, fake writes, and SQLite-in-memory provider tests.
• Documentation that states exactly which guarantees each testing layer provides.

Acceptance criteria

• Application code can construct immutable scalar models and relation graphs without a live database while preserving metadata and key invariants.
• One-to-many, many-to-one, nullable foreign-key, composite-key, missing-key, duplicate-key, empty/single/multiple relation, and direction errors have focused behavior/diagnostics.
• Memory fixtures seed and reset deterministically and reject behavior outside Memory's documented capabilities.
• Fake units of work record writes and mirror the public begin/commit/rollback/failure/cancellation/disposal lifecycle without simulating provider semantics.
• SQLite/server-backed tests remain the documented authority for SQL translation, physical types, defaults, transactions, and provider behavior.
• Consumer-shaped DI replacement examples compile and clearly identify their fidelity level.
• Focused T10 evidence is recorded in 0.10: Produce release evidence and frozen-candidate closeout #106 before this issue closes.

Dependency and order

• W6A may begin after 0.10: Implement native async and cancellation #107 freezes the relevant internal read/cancellation contracts.
• W6B cannot freeze until 0.10: Add DI, hosting, and explicit unit-of-work support #108 defines the production unit-of-work and DI lifetime contract.
• The two lanes may land separately, but this issue closes only when both are complete and integrated.

Explicit non-goals

• A second LINQ-to-Objects query provider or widened DataLinq.Memory semantics.
• Mocks that bypass metadata, key, equality, or relation invariants.
• Simulated SQL-provider transaction behavior.
• Generated test-shape interfaces without demonstrated need.
• A broad query-assertion DSL.

Planning sources

• 0.10 T10 roadmap contract
• Implementation order W6A/W6B
• Durable testing-support design plan