Extract Log Search execution into LogSearcher

[Hirogen]

Summary

Extracts the Log Window's search implementation (Ctrl+F / F3 / Shift+F3) into a pure, table-tested LogSearcher module in LogExpert.Core — Candidate 2 of the LogWindow architecture review, following the pattern proven by the Filter Spread extraction pure module in Core, table-tested in LogExpert.Tests, control delegates.

The ~113-line private search loop in LogWindow.cs is gone. LogSearcher.Find owns direction resolution, the wrap-around-once rule, and regex/ordinal matching over ILogfileReader; the Log Window keeps only the UI half — status-line narration, progress bar, grid scrolling, and cancellation wiring.

Commits / tickets

1. LogSearcher module + 34 tests — Find(params, reader, token, progress) returns a SearchResult (Found / NotFound / Cancelled / InvalidPattern + wrapped flag). Direction truth table, wrap semantics, case modes, regex, cancellation ordering and progress cadence all pinned against a mocked reader — no WinForms needed.
2. Log Window migration — search runs through the module with a CancellationToken (ESC, reload, dead-file, and close all cancel); wrap notices and progress keep their original mid-search timing; the dead "not found" MessageBox is deleted. The shared _shouldCancel flag is untouched for the filter loops.
3. One direction rule — the (IsForward || IsFindNext) && !IsShiftF3Pressed predicate, previously hand-copied in three places, now exists only in LogSearcher.ResolveDirection.
4. Invalid regex narration — a bad pattern reaching a search (e.g. F3 with persisted params) shows the existing localized status-line error instead of dying silently on the background task.
5. Contract & glossary — CONTEXT.md gains the canonical Log Search entry (disambiguated from filtering / FilterSearch); the contract sweep closed one pinning gap (backward wrapped-full-circle).

Plus one drive-by fix found during manual smoke: the Regex Helper dialog crashed on every OK click (Items modified while DataSource was bound) — fixed and regression-tested with three tests replaying the Load→OK sequence.

User-visible changes

• Regex search compiles the pattern once per search instead of once per line (perf).
• Invalid regex patterns report an error instead of hanging on "Searching…".
• A running search is immune to F3 mutating the shared search params mid-flight.
• Regex Helper dialog OK no longer throws ArgumentException.

Everything else is behaviour-preserving, pinned by tests (including cancellation ordering and wrap-notice timing).

Testing

• 34 new LogSearcherTests + 3 RegexHelperDialogTests; full solution suite green (1,394 passed, 0 failed).
• Manual smoke on a large file: ESC cancels a running search; F3/Shift+F3 work through the whole file and across the wrap; not-found narrates correctly.