[BHP1-1375] Add Byram Flame Length to Fire Line Intensity conversion for Spotting

[rjsheperd]

Distance from Active Crown

Purpose

• Adds the conversion from Byram 1959 to the behave C++ lib to enable
  Fire Line Intensity to be calculated from Flame Length (see
  companion PR: [BHP1-1375] Add Byram Flame Length to Fireline Intensity conversion behave#59)
• Adds a new method for building the C++ using Nix Flakes to enable
  more consistent builds (see updates to behave-lib/README.org)

Related Issues

Closes BHP1-1375

Submission Checklist

• Included Jira issue in the PR title (e.g. BHP1-### <title>)
• Code passes linter rules (clj-kondo --lint components/**/src bases/**/src projects/**/src)
• Feature(s) work when compiled (clojure -M:compile-cljs)

Testing

1. Start VMS
2. Run migration migrations.2026-01-20-spot-active-crown-flame-length
3. Sync App
4. Load the worksheet: behave7-BHP1-1375-spot-active-crown-from-flame-length.bp7.zip
5. Verify that the output shows 0.2 miles as the Max Spot Distance
   from Active Crown Fire

[Kcheung42]

Approved!