feat: pretty-printed table output with auto-detected TTY

[vmvarela]

Summary

Implements #156 — when stdout is a TTY, query results are automatically formatted as an aligned table with Unicode box-drawing borders. When piped, output stays CSV (no behavior change for scripts).

Changes

• src/table.zig (new): Table formatting module with two-pass streaming architecture
  • Pass 1: compute column widths and detect numeric columns (O(cols) memory)
  • Pass 2: stream rows directly from SQLite to writer
  • Unicode visual width detection for CJK characters (width 2)
  • Batch write operations for performance
  • NULL values shown explicitly
  • Unit tests for isNumericString and visualWidth
• src/args.zig: Added TableMode enum (auto/always/never) and --table/--no-table CLI flags. --table is validated as incompatible with non-CSV/TSV output formats.
• src/main.zig: Added stdout TTY detection to resolve table mode. Auto-detection restricted to CSV/TSV output formats. Table output disabled when --output writes to a file.
• README.md: Added table output example in the usage section, documented --table/--no-table in the flags table.
• build.zig: Added 7 integration tests covering table output, piped CSV, error paths, numeric alignment, empty results, and --output interaction.

Example

$ printf 'region,product,amount\nAMER,Widget,150.00\nEMEA,Gadget,80.50\nAPAC,Widget,200.00' | sql-pipe --table 'SELECT * FROM t ORDER BY amount DESC'
┌────────┬─────────┬────────┐
│ region │ product │ amount │
├────────┼─────────┼────────┤
│ APAC   │ Widget  │  200.0 │
│ AMER   │ Widget  │  150.0 │
│ EMEA   │ Gadget  │   80.5 │
└────────┴─────────┴────────┘

Unicode support (CJK characters align correctly):

$ printf 'name,city\nAlice,Madrid\nCarol,北京' | sql-pipe --table 'SELECT * FROM t'
┌───────┬────────┐
│ name  │ city   │
├───────┼────────┤
│ Alice │ Madrid │
│ Carol │ 北京   │
└───────┴────────┘

Acceptance Criteria

• When stdout is a TTY, output is formatted as an aligned table with borders
• When stdout is piped, output remains CSV (no behavior change)
• Table includes column headers
• Columns are properly aligned
• Numeric values are right-aligned
• Text values are left-aligned
• Add --table / --no-table flags for explicit control
• Handle wide tables gracefully (columns sized to content)
• Unicode visual width support (CJK characters)
• NULL values shown explicitly
• All existing tests pass
• New tests cover TTY detection, table formatting, and piped output

Code Review Improvements

After initial implementation, code review identified and fixed:

• Blocker: .auto mode now restricted to CSV/TSV output (prevents tables for JSON/XML)
• High: Two-pass streaming reduces memory from O(rows×cols) to O(cols)
• High: sqlite3_step errors now checked and propagated
• Medium: Unicode visual width helper for CJK character alignment
• Low: Batch write operations for performance
• Low: Z023 parameter order compliance
• Low: Unit tests for pure functions