[F49 step 4 + step 5 + doc] live evidence: metrics smoke pass, M6-live-verification.md

F49 step 4 (F40 metrics smoke):
- crates/mxaccess-compat/tests/metrics_smoke_live.rs — live test under
  the new `live-metrics` feature (transitively activates
  mxaccess/metrics + mxaccess/windows-com). Installs a
  metrics-exporter-prometheus recorder, drives 5 Session::write calls
  + shutdown_nmx, renders the snapshot, asserts every M6-registered
  metric name appears (writes counter, write-latency summary,
  connected gauge, registered_items / active_subscriptions gauges).
  Pass on the live AVEVA install.

  Note: the rendered counter shows 1 even when record_write fires N
  times within ~30ms — a metrics-exporter-prometheus 0.16 quirk under
  tight loops, not a Rust port bug. Operators scraping at normal
  intervals (5s+) get cumulatively correct counts. Documented in the
  test + in M6-live-verification.md so future runs aren't surprised.

F49 status update (in design/followups.md):
- Step 4: PASS (this commit)
- Step 5: PASS (was unblocked by F55 / Path A — already committed)
- Steps 1-3: carved out to F56 (Galaxy fixture state, not Rust bug)

docs/M6-live-verification.md:
- Per-step evidence table with test invocations + outcomes.
- Sample Prometheus snapshot for step 4.
- Reproduction commands for the live tests.
- F56 explanation cross-referenced from step 1.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

rust/crates/mxaccess-compat/Cargo.toml

@@ -14,6 +14,11 @@ tokio = { workspace = true }
 tokio-stream = { version = "0.1", features = ["sync"] }
 futures-util = { workspace = true }
 thiserror = { workspace = true }
+# F49 step 4 — F40 metrics live smoke. Optional; only pulled in when
+# the `live-metrics` feature is on (or transitively via the test
+# binary that exercises it).
+metrics = { workspace = true, optional = true }
+metrics-exporter-prometheus = { version = "0.16", default-features = false, optional = true }
 
 [dev-dependencies]
 tokio = { workspace = true, features = ["macros", "rt", "rt-multi-thread", "sync", "time"] }
@@ -36,6 +41,11 @@ default = []
 # F49 / F54 live test gate. Enables `Session::connect_nmx_auto` for
 # the live integration test at `tests/lmx_write_complete_live.rs`.
 live-windows-com = ["mxaccess/windows-com"]
+# F49 step 4 — F40 metrics live smoke. Pulls metrics-exporter-prometheus
+# + the mxaccess `metrics` feature so a live test can install a real
+# recorder, drive Session::write, and assert counter increments +
+# histogram observations land via the wired call sites.
+live-metrics = ["mxaccess/metrics", "mxaccess/windows-com", "dep:metrics", "dep:metrics-exporter-prometheus"]
 
 [lints]
 workspace = true