ci(coverage): replace ReportGenerator with an in-repo report generator

ReportGenerator's free HTML report renders a "Method coverage: Feature is
only available for sponsors" upsell row that can't be cleanly suppressed.
Drop the dependency and render our own.

scripts/coverage_report.py reads the Cobertura XML coverlet already produces
and emits:
- index.html styled like the rest of the Celerity site (per-file line/branch
  bars, uncovered-line ranges linked to source, lowest-coverage-first)
- badge.svg (line-coverage shields-style badge, self-generated)
- summary.md for the PR comment
It also enforces the coverage floor and writes the run summary, folding the
old separate gate step into one script. No third-party branding.

The coverage workflow now runs the script instead of the ReportGenerator
action + inline gate; the README badge points at coverage/badge.svg; the
testing guide documents the generator.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: marius-bughiu

.github/workflows/coverage.yml

@@ -52,13 +52,16 @@ jobs:
           --settings coverage.runsettings
           --results-directory ./TestResults/coverage
 
-      - name: Generate report
-        uses: danielpalme/ReportGenerator-GitHub-Action@v5.5.10
-        with:
-          reports: 'src/TestResults/coverage/**/coverage.cobertura.xml'
-          targetdir: 'coveragereport'
-          reporttypes: 'Html;MarkdownSummaryGithub;Badges;Cobertura;TextSummary'
-          title: 'Celerity coverage'
+      # Renders the HTML report, badge, and PR summary, writes the run summary,
+      # and fails the job if coverage is below the floor — all in one script, so
+      # the report carries the project's own styling and no third-party upsell.
+      - name: Generate report and enforce floor
+        run: >
+          python3 scripts/coverage_report.py
+          --input "src/TestResults/coverage/**/coverage.cobertura.xml"
+          --outdir coveragereport
+          --min-line "$MIN_LINE_COVERAGE"
+          --min-branch "$MIN_BRANCH_COVERAGE"
 
       - name: Upload coverage artifact
         if: always()
@@ -69,39 +72,15 @@ jobs:
           if-no-files-found: warn
           retention-days: 14
 
-      - name: Enforce coverage floor
-        run: |
-          python3 - <<'PY'
-          import os, sys, xml.etree.ElementTree as ET
-          root = ET.parse('coveragereport/Cobertura.xml').getroot()
-          line = float(root.get('line-rate')) * 100
-          branch = float(root.get('branch-rate')) * 100
-          min_line = float(os.environ['MIN_LINE_COVERAGE'])
-          min_branch = float(os.environ['MIN_BRANCH_COVERAGE'])
-          summary = (
-              f"### Coverage\n\n"
-              f"| Metric | Value | Floor |\n|---|---:|---:|\n"
-              f"| Line | {line:.2f}% | {min_line:.0f}% |\n"
-              f"| Branch | {branch:.2f}% | {min_branch:.0f}% |\n"
-          )
-          with open(os.environ['GITHUB_STEP_SUMMARY'], 'a') as f:
-              f.write(summary)
-          print(f"Line coverage:   {line:.2f}% (floor {min_line:.0f}%)")
-          print(f"Branch coverage: {branch:.2f}% (floor {min_branch:.0f}%)")
-          if line < min_line or branch < min_branch:
-              print("::error::Coverage dropped below the configured floor.")
-              sys.exit(1)
-          PY
-
       - name: Comment coverage on PR
-        if: github.event_name == 'pull_request'
+        if: always() && github.event_name == 'pull_request'
         uses: actions/github-script@v7
         with:
           github-token: ${{ secrets.GITHUB_TOKEN }}
           script: |
             const fs = require('fs');
             const marker = '<!-- celerity-coverage-comment -->';
-            const summary = fs.readFileSync('coveragereport/SummaryGithub.md', 'utf8');
+            const summary = fs.readFileSync('coveragereport/summary.md', 'utf8');
             const body = `${marker}\n${summary}`;
 
             const { data: comments } = await github.rest.issues.listComments({

README.md

@@ -1,5 +1,5 @@
 # Celerity
-[![NuGet version (Celerity.Collections)](https://img.shields.io/nuget/v/Celerity.Collections.svg?style=flat-square)](https://www.nuget.org/packages/Celerity.Collections/) [![NuGet version (Celerity.Collections)](https://img.shields.io/nuget/vpre/Celerity.Collections.svg?style=flat-square)](https://www.nuget.org/packages/Celerity.Collections/) [![Live benchmarks](https://img.shields.io/badge/benchmarks-live-0d6e6e?style=flat-square)](https://marius-bughiu.github.io/Celerity/dev/bench/) [![Coverage](https://marius-bughiu.github.io/Celerity/coverage/badge_linecoverage.svg)](https://marius-bughiu.github.io/Celerity/coverage/)
+[![NuGet version (Celerity.Collections)](https://img.shields.io/nuget/v/Celerity.Collections.svg?style=flat-square)](https://www.nuget.org/packages/Celerity.Collections/) [![NuGet version (Celerity.Collections)](https://img.shields.io/nuget/vpre/Celerity.Collections.svg?style=flat-square)](https://www.nuget.org/packages/Celerity.Collections/) [![Live benchmarks](https://img.shields.io/badge/benchmarks-live-0d6e6e?style=flat-square)](https://marius-bughiu.github.io/Celerity/dev/bench/) [![Coverage](https://marius-bughiu.github.io/Celerity/coverage/badge.svg)](https://marius-bughiu.github.io/Celerity/coverage/)
 
 Celerity is a .NET library that provides specialized high-performance collections optimized for specific use cases. It includes data structures designed for better speed or memory efficiency compared to standard .NET collections. The package supports configurable load factors, multiple built-in hash functions, and allows users to define custom hash functions for fine-tuned performance.
 

docs/testing.md

@@ -119,21 +119,21 @@ dotnet test Celerity.Tests/Celerity.Tests.csproj \
   --settings coverage.runsettings \
   --results-directory ./TestResults/coverage
 
-# 2. render an HTML report (one-time tool install)
-dotnet tool install -g dotnet-reportgenerator-globaltool
-reportgenerator \
-  -reports:"./TestResults/coverage/**/coverage.cobertura.xml" \
-  -targetdir:coveragereport \
-  -reporttypes:"Html;TextSummary"
+# 2. render the HTML report + badge (pure Python, no extra tooling)
+python3 ../scripts/coverage_report.py \
+  --input "./TestResults/coverage/**/coverage.cobertura.xml" \
+  --outdir ../coveragereport --min-line 95 --min-branch 90
 
 # 3. open coveragereport/index.html
 ```
 
+The report is rendered by [`scripts/coverage_report.py`](../scripts/coverage_report.py) — a small generator that reads the Cobertura XML coverlet produces and emits an `index.html` styled like the rest of the Celerity site, a `badge.svg`, and a `summary.md`. It exists so the report carries the project's own look and no third-party "sponsors only" upsell; there is no dependency on ReportGenerator.
+
 ### CI gate
 
 The `coverage` workflow (`.github/workflows/coverage.yml`) runs on every PR and on `main`:
 
-- Collects coverage, renders the report with ReportGenerator, and uploads it as a build artifact.
+- Collects coverage, renders the report + badge with `scripts/coverage_report.py`, and uploads it as a build artifact.
 - **Fails the build** if line coverage drops below `MIN_LINE_COVERAGE` (95%) or branch coverage below `MIN_BRANCH_COVERAGE` (90%). The suite sits far above these (~99.9% line) — the floor guards against silent regressions; it is not the target.
 - Posts a coverage summary comment on the PR.
 - On `main`, publishes the HTML report to `gh-pages` under [`/coverage`](https://marius-bughiu.github.io/Celerity/coverage/) and refreshes the README badge.