Add test coverage for TimeSpreadCalculator

[Hirogen]

Background

Timestamp Locator extraction, merged in #664 rewired TimeSpreadCalculator off ILogWindow and onto the pure TimestampLocator + ITimestampSource seam. That was the blocker to testing it — the calculator no longer needs a WinForms LogWindow to run, only a fake ITimestampSource, the same seam TimestampLocatorTests.cs already exercises.

It remains untested. DoCalc and DoCalcViaTime (the two spread algorithms — line-mode and time-mode) have no coverage, and DoCalcViaTime carries a pre-existing //TODO Refactor this method marker of its own.

What to build

Test coverage for TimeSpreadCalculator's two calculation paths (DoCalc, DoCalcViaTime) against a fake ITimestampSource + TimestampLocator, in the same style as src/LogExpert.Tests/Timestamp/TimestampLocatorTests.cs. The worker-thread/event-driven surface (WorkerFx, _calcEvent, _lineCountEvent) needs its own think about whether to test through it or extract the calculation from the threading — do that thinking as part of this ticket, not before it.

Acceptance criteria

• DoCalc (line mode) has test coverage: empty file, single line, no timestamps, uniform spacing, irregular gaps
• DoCalcViaTime (time mode) has equivalent coverage
• Tests run without instantiating any WinForms type
• Solution builds; all existing tests still pass

Blocked by

None — TimeSpreadCalculator is already rewired onto the testable seam.