Benchmark: support -c correctly

[mgravell]

give each client its own connection unless +m specified

readme.md documents +m/-m as "when enabled clients share a connection, otherwise each client has a separate connection". The classic (old-core) leg did not implement the second half: OldCoreBenchmarkBase built a single ConnectionMultiplexer in its constructor and GetClient(int index) ignored the index entirely, returning that one IDatabase to every worker. Multiplexed was never read on this leg at all -- +m parsed and then did nothing.

So -c was CONCURRENCY (in-flight requests pipelined onto one connection), not connection count, on the leg that ships. The fan-out above it was always correct: BenchmarkBase allocates ClientCount tasks and calls GetClient(i) per worker; only the worker-to-connection mapping collapsed.

A multiplexer is one connection per endpoint by design, so "a connection per client" means a multiplexer per client. Now builds Multiplexed ? 1 : ClientCount of them and maps worker i to i % connectionCount, so +m reproduces the previous behaviour exactly and is the only way to get it. NewCoreBenchmark already did this (new(count: Multiplexed ? 1 : ClientCount)); BridgeBenchmark shares this base and inherits the fix.

VERIFIED against a live server, sampling DURING the run from two independent vantage points -- the OS TCP table by owning pid, and the server's own CLIENT LIST:

-c 1 OS 1,1,1,1 CLIENT LIST 1 (+ the cli itself)
-c 8 OS 8,8,8,8 CLIENT LIST 8
-c 50 OS 50,50,50,50 CLIENT LIST 50
-c 50 +m OS 1,1,1,1 CLIENT LIST 1

The +m cell is the discriminating one: without it, a fix that simply hard-wired N connections would pass just as happily. --basic also exercised (exit 0).

AND A BANNER THAT CANNOT AGREE WITH A FLAG THAT DID NOTHING. The per-test banner printed ", mux" whenever +m was passed -- including on the leg where it had no effect -- so it reported the FLAG, not the behaviour, and could not have caught this. ConnectionCount is virtual, defaulting to the intended count, and OldCoreBenchmarkBase overrides it with _connectionMultiplexers.Length: the number actually created. It is printed on the ### ... ### announce line, which survives -q so a harness can gate on it, and in the per-test banner.

Checklist

• I fully and freely contribute this code in accordance with the project license (and am legally able to do so)
• I take responsibility for this contribution's quality and correctness, including any portions produced with AI assistance (see CONTRIBUTING.md).