fix: skip 'Game starting' message when resuming a saved game

[njbrake]

Summary

• When resuming a saved game, the "Game starting..." message no longer appears in the game log. The log already contains the full event history from the save file, so this message was redundant and confusing.
• Detects resume by checking whether the game phase is past Setup, then sends an empty initial state update (TUI still gets the board state to render, just no log entry).
• Adds two regression tests: one proving resumed games skip the message, one proving fresh games still show it.

Investigation notes

Audited the full save/resume round-trip. All gameplay state (board, buildings, roads, resources, dev card deck order, robber position, longest road/army, friendly_robber, ports, phase, turn tracking) survives serialization correctly. The current player is always preserved accurately at turn boundaries.

Closes #178

Test plan

• cargo test -- 436 tests pass (434 existing + 2 new)
• cargo fmt and cargo clippy clean
• Manual: start a game, quit mid-game, resume via Continue -- verify no "Game starting..." in log
• Manual: start a fresh game -- verify "Game starting..." still appears

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