Tests for plotHistoricFires(), plotCumulativeBurns() and plotBurnSummary()

[eliotmcintire]

Adds tests/testthat/test-plotSummaries.R. The three plot functions run on made-up replicate outputs (burn summary CSVs and burn maps), and the tests check the figure files they write. A read error is reported as it happens, and .attributes() handles each spatial class. Measured locally with covr, R/plot_summaries.R goes from 14% to 97%. This PR is tests only.

Stacked on #91: merge #91 first.

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 59.61%. Comparing base (d20bc2d) to head (fee2deb).
⚠️ Report is 8 commits behind head on development.

Additional details and impacted files

@@               Coverage Diff               @@
##           development      #94      +/-   ##
===============================================
+ Coverage        53.39%   59.61%   +6.21%     
===============================================
  Files               40       40              
  Lines             3798     3798              
===============================================
+ Hits              2028     2264     +236     
+ Misses            1770     1534     -236     

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[eliotmcintire]

Folded into #95, whose branch already contains these commits, to cut CI runs.