fix: Sync cumulative game clock with period clock display

[JRemitz]

Summary

• Fixes the 1-second drift between the cumulative game clock and the period clock (Cumulative game clock #16)
• Fixes cumulative game clock resetting to 0:00 on OBS restart
• Bumps version to 0.7.1

Details

Clock sync fix: Both clocks internally track tenths but display whole seconds. Previously, each truncated tenths / 10 independently, so floor(a/10) + floor(b/10) could differ from floor((a+b)/10) — creating a 1-second gap. The fix derives cumulative elapsed as period_length - displayed_period_seconds, guaranteeing displayed_period + cumulative == period_length at every tick.

Persistence fix: cumulative_clock.txt was written but never read back on startup. Now scoreboard_read_all_files() parses it (both M:SS and H:MM:SS formats) and restores game_clock_accumulated_tenths by subtracting the current-period elapsed time from the total.

Test plan

• test_game_clock_subsecond_sync — verifies invariant with sub-second ticks
• test_game_clock_subsecond_sync_across_periods — verifies across period boundaries
• test_game_clock_read_all_files — write/reset/read round-trip
• test_game_clock_read_all_files_across_periods — multi-period round-trip
• test_game_clock_read_disabled_no_restore — ignored when feature is off
• test_game_clock_read_hmmss_format — H:MM:SS parsing
• test_game_clock_read_invalid_format — garbage file silently ignored
• test_game_clock_read_clamps_negative — negative accumulated clamped to 0
• All 7 test suites pass, 100% line coverage
• Manual: enable cumulative clock, run a period, close/reopen OBS, verify clock is restored

Closes #16

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