Expose encoding selection/detection in the CLI

[bytebrujo]

Problem

The Zig iconv layer (src/zig/iconv.zig) supports transcoding and csv_detect_encoding, but the CLI surfaces no --encoding flag and has no detect command.

Proposal

• Add --encoding flag across read commands
• Add turbocsv encoding detect data.csv

Acceptance

• --encoding honored by count/head/select/etc.
• detect command reports encoding
• Tests with non-UTF8 fixtures