* fix(proxy): sum inference counters across ProxyClient instances

Each ProxyClient writes its own running totals to the shared stats file,
but the reader kept only the last line per problem, so agents that build
a client per LLM call reported one call per problem.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

* fix(proxy): scope summed counters to one execution

/run and /run_matrix can evaluate the same problem repeatedly against one
shared stats file. Tag each execution's lines with an id set in the child
process and read back only that execution's lines.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

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Co-authored-by: Claude Opus 5.5 (1M context) <noreply@anthropic.com>