Time Control Tag Support

## Problem

Time control tags are ignored during PGN import:
- `[TimeControl]`
- `[WhiteClock]`
- `[BlackClock]`

## Solution

These should be stored as first-class fields on `Game` or at minimum preserved in the generic tags list during import.

## File

`src/pgn.rs` (`parse_tag()`)

## Priority

Medium - improves PGN completeness