Add proposed table validation design and update docs navigation

[0x5143]

Motivation

• Provide a unified, extensible design for generation-time data validation across all table types to surface semantic and structural errors earlier and with richer diagnostics.
• Define a TableDataSnapshot/four-state value model, phased validation passes, a single TableTypeDescriptor registry and a versioned diagnostics contract to avoid duplicated logic and maintain compatibility.
• Capture a phased rollout plan (P0–P3) and recommended constraints to ensure migration safety and observable behavior before implementing runtime changes.

Description

• Add a new design document docs/designs/table-validation-design.md that describes goals, architecture, validation phases, API sketches (ITableValidationPass, IProjectValidationPass, TableValidationContext, snapshot model), diagnostics contract, and a multi-phase implementation plan.
• Update docs/README.md to include the new design in the recommended reading and the docs/ directory listing by adding an entry and link to table-validation-design.md.
• Update docs/guides/table-types.md to reference the new design and note that the validation scheme is proposed and not yet a released rule syntax.

Testing

• This PR contains documentation changes only and no production code modifications, so there are no unit or integration tests affected.
• No automated behavioral tests were added or changed for this PR; documentation linting and link consistency checks were the relevant validations if run in CI.
• Any future implementation work will include the registration, layout, type, structure, project, and regression test categories outlined by the design document.

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Codex Task