Apply source map section columns only to their first line

[OskarEichler]

Summary

Index source map column offsets apply only to the first generated line of each section, while line offsets apply to all lines.

Matches ECMA-426 DecodeIndexSourceMap: column offset applies to the first generated line only. Sprockets represents generated lines starting at 1.

Reproduction

With require "sprockets"; require "tmpdir":

map = {"version"=>3, "file"=>"a.js", "sources"=>["a.js"], "names"=>[], "mappings"=>"AAAA;AACA"}
index = {"version"=>3, "sections"=>[{"offset"=>{"line"=>5,"column"=>7}, "map"=>map}]}
p Sprockets::SourceMapUtils.decode_source_map(index)[:mappings].map { |m| m[:generated] }
# [[6,7], [7,0]]; previously [[6,7], [7,7]].

Verification

• Isolated patch and release baseline: unchanged bundle exec rake test, 913 tests /4,109 assertions /zero failures/errors /four skips, Ruby 3.2.11 and 4.0.6. 54 focused external checks for this patch on each Ruby.
• Eight follow-ups combined with existing Use Ruby 3 keyword argument handling for find_asset! #792: same suite plus 1,540 focused checks per Ruby. Combined isolated test task also passes. Full RuboCop passes all 133 files; Ruby syntax and diff whitespace checks pass.
• Gem build preserves 84 file paths, version 4.4.1, Ruby >=2.5 and dependency metadata. Consumer based on installed release/current main 49d9f20c00ac7d9a7ee57db058961edcccd86699, without unrelated upgrades.
• Existing asset-clean test is timing-sensitive: touching its fixture within the current timestamp second reproduced the same line-80 assertion failure on baseline and an independent cache patch (2/4 each). Full successful runs above are actual runs; no test code was changed. Normal skips cover legacy Ruby, Windows and two Sass cases; non-terminal stdin adds the Unix-device skip.

Compatibility and limitations

Breaking changes: no intentional change to valid supported behavior; the incorrect behavior above changes. No new dependencies or version upgrades. Older Ruby/Linux/JRuby checks are left to upstream CI, not claimed locally. No tests added/edited under the consumer repository's no-test-edit policy; focused evidence was run externally. Prepared with AI assistance. This is a focused correction, not an exhaustive Sprockets review.