bench: run on Linux only, and name the jobs for what they measure (#8)

Windows and macOS were carried on an argument that does not survive
examination. They were the only place in the whole chain that installed the
published wheel and ran it, which made this benchmark an install smoke test by
accident. That check is worth having, and worth forty seconds next to the build
in release.yml rather than thirteen minutes inside a performance measurement:
nobody reads a benchmark to find out whether a package imports. release.yml
already builds on all four targets, it just never executes what it built.

What is lost is a per-platform timing that was never quoted; the numbers that
get published are the Linux ones. Adding a platform back is one matrix entry.

The jobs are also named for what they measure rather than for the runner they
landed on, which the row already says: `backtests (ubuntu-latest)` and `grids
(ubuntu-latest)` instead of a raw label next to a hand-written one.

.github/workflows/bench-vs-vectorbt.yml

@@ -42,7 +42,10 @@ concurrency:
 
 jobs:
   bench:
-    name: ${{ matrix.os }}
+    # Named for what the job measures, not for the runner it landed on. The
+    # runner is already on the row; what a reader needs from the job list is
+    # which half of the benchmark it is.
+    name: backtests (${{ matrix.os }})
     runs-on: ${{ matrix.os }}
     # A published release is not always a released version. The preview tags
     # 0.18.0rc1 and rc2 went to the public repository without ever reaching
@@ -56,22 +59,19 @@ jobs:
       fail-fast: false
       matrix:
         include:
-          # Three lengths, not five. 10k was sub-millisecond on the engine
+          # Linux only, on purpose. The other two runners were carried for a
-          # side, which measures the clock rather than the work, and 5M sat
+          # reason that does not survive examination: they were the only place
-          # between two points that already bracket it. What is left is a
+          # anything installed the published wheel on Windows or macOS, which
-          # decade apart each step, which is what makes the trend readable.
+          # made this benchmark a smoke test by accident. That check is worth
+          # having and worth 40 seconds next to the build in release.yml, not 13
+          # minutes inside a performance measurement, and nobody reads a
+          # benchmark to find out whether a package imports.
+          #
+          # What is lost is a per-platform timing, which was never quoted: the
+          # numbers that get published are the Linux ones. Adding a platform
+          # back is one entry here.
           - os: ubuntu-latest
             bars: "100000 1000000 10000000"
-          - os: windows-latest
-            bars: "100000 1000000 10000000"
-          # macOS runners ship 7 GB of RAM against 16 GB elsewhere, and vectorbt
-          # materialises the simulation in memory (roughly 150 MB per million
-          # bars, measured). The top size is trimmed so a point is never lost to
-          # swapping, which would time the disk instead of the engine. The same
-          # arithmetic is why 10M bars is added on the other two and not here:
-          # measured, that point adds 1.55 GB on vectorbt's side alone.
-          - os: macos-latest
-            bars: "100000 1000000"
 
     env:
       PYTHONUNBUFFERED: "1"
@@ -152,7 +152,7 @@ jobs:
   # Parameter grids, which need a licence and therefore a job of their own
   # ------------------------------------------------------------------------ #
   sweeps:
-    name: sweeps (ubuntu)
+    name: grids (ubuntu-latest)
     runs-on: ubuntu-latest
     if: github.event_name != 'release' || github.event.release.prerelease == false
     timeout-minutes: 45