The find_source_record lambda was doing a reverse linear scan through
the entire source map for every instruction emitted, resulting in
quadratic behavior. This was catastrophic for large scripts like
Octane/mandreel.js, where compile() dominated the profile at ~30s.
Since both source map entries and instruction iteration are ordered by
offset, replace the per-instruction scan with a forward cursor that
advances in lockstep with instruction emission.
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