fix(status): count threads as they arrive instead of "Searching..."

Item 74. runQuery() set the status bar once and only queryFinished cleared
it, so the bar kept claiming a query was running for the whole walk while
rows were visibly arriving behind it. Measured cold against a 1.1 GB index:
the first batch reaches the model at 642 ms and the walk finishes at 5714 ms,
so five seconds of a slow query read as a frozen one.

onThreadsReady now sets the bar from the model's own row count after each
batch, which is the number of rows the user can actually see. No timing
changes; this only stops the bar from lying.

The refresh branch returns before the new line, so a background refresh stays
silent exactly as onQueryFinished already keeps it. That silence has its own
test, which fails when the write is moved above the guard.

beginRefreshForTesting() is a new seam: refreshCurrentQuery() returns early
without a worker and a bare window has none, so a test cannot otherwise reach
the refresh path.

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