blogex: coverage floor (74) + test-scaffolding exclusions

`test_coverage` sets the floor the Lens pass enforces, measured first: 74.53%
with test/support excluded (`mix test --cover` and the exported sample give the
same number). The rule — a FLOOR, ratcheted up as behavior yaks land, never
lowered — is written next to the config, along with why each module is excluded
(test scaffolding only: fixtures measure the tests, not the library).

Wiring, per the fast/push split: `test.coverage` runs the suite once with
`--cover --export-coverage default`; the `coverage` sensor then enforces the
floor from that export via `mix test.coverage` (no second suite run). So
`verify` runs the suite first and the sensors after it. The sensor guards on the
export existing: `mix test.coverage` exits 0 when it finds no .coverdata, and a
floor check that did not run must not report success.

blogex/.gitignore: cover/ (generated).

blogex/mix.exs

@@ -13,6 +13,7 @@ defmodule Blogex.MixProject do
       start_permanent: Mix.env() == :prod,
       aliases: aliases(),
       deps: deps(),
+      test_coverage: test_coverage(),
       docs: docs(),
       package: package(),
       description: "A multi-blog engine powered by NimblePublisher for Phoenix apps"
@@ -49,6 +50,30 @@ defmodule Blogex.MixProject do
     ]
   end
 
+  # The coverage floor the Lens pass enforces (`make verify` -> `make coverage`
+  # in blogex/). Rule: the threshold is a FLOOR. Raise it as behavior-test yaks
+  # land; never lower it. A drop means the change (or an exclusion) is wrong.
+  #
+  # Measured 2026-10-26, with the test scaffolding below excluded: 74.53% — the
+  # same via `mix test --cover` and via `mix test --cover --export-coverage
+  # default` + `mix test.coverage`. Floor is the integer below that.
+  #
+  # Excluded: the test scaffolding (Blogex.Test.*) only. Fixtures measure the
+  # tests, not the library. Everything in lib/blogex counts, and today's gaps
+  # are real: Blogex (the delegating facade), Blogex.Components and
+  # Blogex.Layout are at 0% because the router tests only exercise the JSON
+  # responses — moving this floor up is the behavior-test work.
+  defp test_coverage do
+    [
+      summary: [threshold: 74],
+      ignore_modules: [
+        Blogex.Test.FakeBlog,
+        Blogex.Test.PostBuilder,
+        Blogex.Test.Setup
+      ]
+    ]
+  end
+
   defp docs do
     [
       main: "readme",