ci: add branch coverage to lcov

Enable branch coverage with --rc lcov_branch_coverage=1.
This provides more detailed coverage metrics:
- Line coverage: % of code lines executed
- Function coverage: % of functions called
- Branch coverage: % of if/else/switch branches taken

Branch coverage is a step towards MC/DC (Modified Condition/Decision Coverage)
used in safety-critical systems.

Also add coverage/ to .gitignore.

.gitignore

@@ -107,3 +107,4 @@ breaks
 *.tar.*
 *.zip
 *.log*
+coverage/