…ndings

Address code review findings:

- Derive MaxPending from the per-node sum of subject volumes with local
  subscribers, since MaxPending is a per-connection budget and one
  subscribe connection carries every coalesced subscription on its
  node. The previous per-subscriber derivation undersized multi-subject
  nodes.
- Derive the per-subscription pending byte limit (new LocalQueueBytes
  knob) alongside the message limit; previously the 512 MiB default
  could trip before the derived message limit.
- Pad message-count budgets with probe headroom so in-flight readiness
  probes cannot consume capacity sized for the benchmark burst.
- Warn when the derived local queue hits its cap and can no longer
  guarantee a drop-free run.
- Return partial Results on publish and flush errors for diagnostics,
  matching the documented Run contract.
- Register subscriber cleanup before subscribing so partial subscribe
  failures are cleaned up by the workload itself.
- Remove a no-op subscriber-node flush whose comment misattributed the
  interest guarantee; SubscribeWithErr flushes the SUB itself.
- Record effective (overridden) configs in matrix report rows.

- Replace the env-gated TestBenchMatrix test with a cmd/natsbench CLI:
  no flags runs the default matrix, -scenario runs one named scenario,
  and shape flags (-payload/-subjects/-publishers/-subscribers/
  -replicas) run a custom configuration. Markdown goes to stdout, logs
  to stderr, and a failed run exits nonzero. The report-to-file env var
  is gone; redirect stdout instead.
- Remove Config.withDefaults: Run now requires a fully populated
  config and validates that Timeout is positive. The CLI defaults the
  timeout to 2 minutes.
- Collapse the readiness gate's two plan inversions into a single
  subjectNodes mapping that serves as both the probe schedule and each
  subscriber's required probe set.
- Document why startPublishers parks on a barrier and when the
  zero-expectation pre-close of allDone applies.
- Drop digit-separator underscores from numeric literals.

- Encode readiness probes as a sentinel byte plus the decimal node
  index instead of a BigEndian uint64, dropping the encoding/binary and
  math dependencies and the overflow guard.
- Return publisher errors over a buffered, closed-on-completion channel
  instead of writing into a shared slice, removing any question of a
  data race on the error collection.
- Move the CLI driver into the natsbench package as the exported Main
  plus a testable cliRun.scenarios; cmd/natsbench is now a thin
  entrypoint. Adds unit coverage for scenario selection.
- Expand the plan doc comment with a concrete worked example of the
  publisher/subscriber to subject/node assignment and expected counts.

Replace the 0x5b sentinel byte with a 'natsbench-probe:' string prefix.
Both distinguish probes from the all-zero benchmark payloads equally
well, but the prefix is self-documenting in packet captures and
debuggers. Decode with strings.CutPrefix.

Collapse the library and its cmd/natsbench entrypoint into a single
package main with a main() that calls runCLI. The benchmark is now run
directly with 'go run ./coderd/x/nats/natsbench/'. Tests still live in
the same directory and continue to pass.

…comments

- Rename awaitReadiness -> awaitTopologyReady, readinessConverged ->
  isReady, readinessShortfall -> unreadySubscribers.
- Give each plan field its own comment line.
- Note why probe flushing dedupes pubNode (it is indexed by publisher,
  so multiple publishers share a node).

Compute the sorted distinct publisher and subscriber node sets once in
buildPlan (plan.pubNodes / plan.subNodes) instead of recomputing
uniqueInts at each call site, including on every iteration of the
readiness gate loop. Several publishers or subscribers can share a node,
so per-node work (flushing, burst sizing) needs the deduped set.

Drop the redundant Scenario column (the payload group header and the
Replicas column already identify each row) and the always-zero Drops
and always-empty Notes columns. A Status column is now included only
for groups that contain an invalid run, so clean matrices render as a
compact four-column table.

Pad every table cell to its column's widest value so the raw markdown
also lines up in a fixed-width terminal, instead of relying on a
markdown viewer to align ragged pipes. Numeric columns stay
right-aligned and the Status column is left-aligned.

The standard matrix now runs with 3 publisher and 3 subscriber
connections (DefaultConns) to match the prior natsbench harness, which
spreads same-subject hashing across connections and raises single-node
throughput over the production 1/1 default. New -publish-conns and
-subscribe-conns flags apply to every run, so 1/1 production behavior is
still reproducible with -publish-conns 1 -subscribe-conns 1.

Drop the trailing colons from table separator rows. Cells are already
padded for terminal alignment, so the GitHub markdown alignment hints
added visual noise without changing the rendered terminal output.

Add Subjects, Publishers, and Subscribers columns to the report so the
workload shape is explicit. The default matrix holds these constant, but
named-scenario overrides and custom runs vary them, and a table that
hides the shape is easy to misread.

probeNode ran string(payload) on every delivered message, allocating a
full copy of the (up to 64 KiB) payload per delivery. At high fan-out
this dominated runtime via GC pressure and understated throughput by up
to ~10x. Compare the probe prefix as bytes against a package-level byte
slice and convert only the tiny trailing node index to a string, and
only for actual probes, so benchmark payloads cost no allocation.