Redesign fluent API with explicit table selection

[jogibear9988]

The unreleased fluent API mixes object names and table names in positional arguments, making migrations hard to read and easy to misconfigure. Replace those overloads with staged expressions that make the target table and the next supported step explicit.

migration.Create.Column("Email").OnTable("Users").AsString(320);
migration.Alter.Column("Name").OnTable("Users").AsString(200);
migration.Rename.Column("Name").OnTable("Users").To("DisplayName");
migration.Delete.Column("Email").FromTable("Users");

migration.Create.Index("IX_Users_Email")
    .OnTable("Users").WithColumns("Email").Unique();
migration.Create.ForeignKey("FK_Orders_Users")
    .FromTable("Orders").WithColumns("UserId")
    .ToTable("Users").WithColumns("Id")
    .OnDelete(ForeignKeyConstraintType.Cascade);

API changes

• Apply explicit table selection to columns, indexes, constraints, foreign keys, views, renames, removals, data transfers, and schema reads. Remove the old positional fluent overloads.
• Share column type/options methods across table definitions, additions, and alterations. Each column handle stays bound to its own column; named fluent columns require an explicit type.
• Split insert, update, and delete into distinct builders exposing only valid steps. Each insert describes one row. Updates and deletes must finish with a predicate or explicit AllRows().
• Register expressions in authoring order and reject incomplete chains during Build, Apply, or Preview, before any queued operation executes. Reject duplicate completion and retain independent snapshots of mutable definitions.
• Rename IfDatabase to IfProvider; move table selections to Schema.Table(table).Select(...) / .SelectScalar(...).
• Make AsDateTime() map to DbType.DateTime; add AsDateTime2() for the former mapping.

Execution fixes and documentation

Correct provider overload selection for unfiltered updates and scalar reads, and quote table names for scalar reads and unfiltered deletion. Add regression coverage for incomplete chains, provider dispatch, column handles, snapshots, composite foreign keys, quoted names, and SQLite schema/data changes.

Update the README, manual sources, generated website and search index, capability inventory, breaking-change guide, and runnable fluent quick start. The quick start now previews, applies, and automatically reverses an added column and index.

This intentionally breaks the unreleased fluent API. The migration guide documents the replacement syntax and behavior changes. This PR is based directly on master and contains only the fluent API work.

Validation

• Solution build passed.
• Unit suite: 584 passed, zero skipped.
• SQLite suite: 306 passed, zero skipped.
• 106 published C# samples compiled, with 8 Classic/Fluent SQLite parity checks passing.
• Generated documentation, local links, anchors, search entries, and paired examples verified, including after rebasing onto master.
• Fluent quick-start preview, migration, and automatic rollback passed.
• External database suites were not run locally.

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