Fix post-increment causing silent exit under set -e

[objctp]

Background

Related #618

LCOV and HTML coverage reports were incomplete/missing due to ((var++)) post-increment operator causing early script termination under set -e.

Changes

• Fixed report_lcov() and generate_file_html() post-increment causing silent exit under set -e
• Standardised remaining ((var++)) to ((++var)) for consistency (preventive)

Checklist

• I updated the CHANGELOG.md to reflect the new feature or fix
• I updated the documentation to reflect the changes

[Chemaclass]

• Complete coverage: all standalone ((var++)) outside for loop headers are changed. state.sh is correctly left alone since it already uses || true.
• Correct scope: bashunit runs with set -euo pipefail (set in /bashunit line 2 and src/globals.sh), so this affects bashunit's own execution, not just user scripts.
• CHANGELOG entry is accurate and references #618.
• No behavior change: only the expression return value changes, not the increment itself.

Concern: No regression test

The PR has zero tests. Given this bug causes silent failures (the { ... } > "$output_file" redirect swallows the error), a regression test would be valuable. Something like:

function test_get_executable_lines_succeeds_under_set_e() {
  local result
  result=$(set -e; bashunit::coverage::get_executable_lines "some_file.sh")
  assert_greater_than 0 "$result"
}

This would prevent future regressions if someone reintroduces ((var++)).

[Chemaclass]

The failing are flaky tests not related to this changes. PR good to merge. Thanks!