refactor: extract format abstraction and modularize source

[vmvarela]

Summary

Introduce src/format.zig as the canonical module for format types and output dispatch, and decompose the 2308-line main.zig god file into focused modules.

Motivation

main.zig mixes CLI parsing, business logic, SQL DDL, loading orchestration, and output dispatch. Adding a new format currently requires touching 6+ places in main.zig. The goal is to reduce that to: create one new file + register in one place.

Phases

• Phase 1 (this issue): Create src/format.zig with InputFormat, OutputFormat, ColumnType, LoadOpts, WriteOpts, and OutputWriter. Migrate execQuery to use OutputWriter.
• Phase 2: Reorganize into src/formats/ and src/modes/ subdirectories.
• Phase 3: Consolidate sqlite.zig helpers (eliminate duplicates with main.zig).
• Phase 4: Extract loader.zig to unify the 4 duplicated CSV-loading loops.
• Phase 5: Move integration tests out of build.zig.