The classic leg used ONE connection whatever -c said; give each client its own (#3170)

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.