fix: support digits in clan chat names + optional default clan chat for new players

[HarleyGilpin]

Fixes

Clan names containing digits were broken: writeLong(String) in JagExtensions.kt encoded names as base-37 RS longs but checked digit characters against in 0..9 instead of their ASCII codes (48..57), so digits were silently dropped.

• Mixed names (e.g. Clan123) displayed without their digits.
• All-numeric names (e.g. 123) encoded to 0, making the channel impossible to join.

One-line fix to the character range; the - 21 offset was already correct ('0' → 27 … '9' → 36).

Features

Added an optional world.start.clanChat setting (game.properties). When set to a channel owner's display name, brand-new accounts automatically join that clan chat on first login. Detection uses the transient new_player flag set by
AccountManager.create(), so it is unaffected by script ordering or the world.start.creation setting. Bots are excluded, and the join persists via the existing clan_chat variable (normal auto-rejoin/leave behaviour applies).

Tests

• JagExtensionsTest: digit encoding cases (clan123, all-numeric 123).
• ClanTest: new players auto-join the default channel; existing players don't.
• WorldTest.createPlayer gained an optional pre-spawn setup lambda for variable injection.