Stream perfetto source fetches via per-block callbacks

Replace the *_for_perfetto_sql() -> Result<String> methods (built the
SQL in one place, executed it in another) with *_for_perfetto(args,
on_block) that encapsulate the SQL and stream through
execute_for_each(), the same model as get_query_logs(). execute_into()
is deleted - execute_for_each() is the single streaming primitive now.

The bounded channel in the worker stays: it is what serializes the
concurrently-fetched sources into the single &mut PerfettoTraceBuilder
consumer. stream_perfetto_source() shrinks to the error-tolerance
wrapper (skip a source on UNKNOWN_TABLE/CANNOT_EXTRACT_TABLE_STRUCTURE)
around the fetch future, and apply_via() builds the per-block callback
that defers applying the block by sending it into the channel. The
separate "Failed to build query" log path is gone - SQL-build errors
now surface through the fetch result like any other error.

Integration tests collect the streamed blocks via the fetch_streamed!
macro (fixture results fit in a single block, keeping the assertions
unchanged); find_rows() becomes generic since streamed blocks are
Block<Simple>, not the Block<Complex> of fetch_all().

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

Author: azat