fix(coverage): cover every line of a multi-line statement

[Chemaclass]

🤔 Background

Related #1338

Bash reports one executed statement to the DEBUG trap on a single line, even when it is written over several. #722 handled the backslash chain; every other way a statement spans lines was left out, so an array literal written one element per line cost one uncovered line per element. On Bash 3.2 the assignment is reported on its closing ), which is non-executable, so the hit was discarded and the whole array read as uncovered.

💡 Changes

• Group source lines into statement spans and give every line of a span the highest count recorded in it, so a statement that ran is covered wherever the trap attributed it. Array literals, multi-line strings and heredoc bodies are now covered; the #722 backslash chain is unchanged.
• A multi-line $( ) is deliberately left alone: its interior lines are commands tracked in their own right, and crediting them would report lines that never ran.
• One scanner, a Bash reference mirrored in awk, replacing five copies of the old propagation loop. The differential now compares the scanner too over every repo shell file, and asserts each one lexes to a clean end state.
• Coverage rises where a multi-line statement really did run, and the report phase is 20-30% slower (3-5% of a whole --coverage run) — the cost of a per-line lexer. Verified on Bash 3.0, 3.2 and 5.3.