fix(coverage): isolate nested command spans

[Chemaclass]

Background

Follow-up to #1339. A multiline array, quote, or continued command that contains a command or process substitution can lose coverage on its outer literal segments or mark skipped child commands as covered. Case arm delimiters and arithmetic syntax can also corrupt the scanner state and affect later statements.

Changes

• Preserve parent literal and continuation spans across substitutions while excluding child-only command lines.
• Track case header, pattern, and body phases, arithmetic contexts, heredocs, and continuation ancestry consistently in the Bash and awk scanners.
• Add opening-hit, closing-hit, nested case, arithmetic, heredoc, and skipped-child regressions with terminal, LCOV, and HTML report coverage.

Validation

• 60 focused tests and 123 assertions passed.
• ShellCheck passed on all changed shell files.
• Bash syntax checks passed, including the regression fixtures.
• make lint and git diff --check passed.

Checklist

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

Refs #1338