Preserve multiline inline if chains

[adamcavendish]

Summary

• preserve source newlines before inline $if where they represent multiline inline fragments
• keep inline $else_if / $else attached when chains span source lines
• retain assignment-continuation collapse for value expressions like const mode =\n $if(...)
• add exact coverage for pipe continuations, multiline siblings, split chains, comma guards, and import tracking
• fix inline-control-flow example wording about evaluation timing

Verification

• cargo test
• just lint
• cargo fmt --check
• git diff --check
• cargo run --example inline_control_flow