Fix single-quoted strings with embedded newlines, and make heredoc body parts lazy-loadable

[jdiamond]

This fixes two related bugs discovered while writing tests for single-quoted strings containing backticks and $().

Bug 1: single-quoted strings with embedded newlines were parsed incorrectly

computeWordParts had a heuristic — if word.text contained a newline, route to buildHereDocParts (heredoc scanning). The intent was to handle heredoc bodies, but any single-quoted string with a literal newline also triggered this path. The result: backticks and $() inside a multi-line single-quoted string were expanded as real shell substitutions instead of treated as literals.

Fixed by splitting the two responsibilities: computeWordParts always uses buildWordParts (correct shell-word semantics), and a new computeHereDocBodyParts handles heredoc scanning explicitly.

Bug 2: redirect.body.parts always returned undefined

Heredoc body words were constructed without a source string, so word.parts returned undefined immediately — consumers had no way to access the structural parts via the normal API.

Fixed by wiring heredoc body words into the lazy parts system. WordImpl now accepts an optional per-instance resolver at construction time (alongside the existing static default), and heredoc body words are constructed with computeHereDocBodyParts as their resolver. redirect.body.parts now works as expected.

New tests cover both: multi-line single-quoted strings with backticks and $(), and the existing heredoc expansion tests now validate through word.parts rather than calling the internal function directly.

[jdiamond]

Heads up: PR #2 also modifies parts.ts. If both are merged, the merge commit will need a test run — I hit a silent integration issue where git combined the two parts.ts changes without conflict but moved a resolution loop into the wrong function. The tests caught it. Working fix is on https://github.com/jdiamond/unbash/tree/combined-fixes. Happy to submit a follow-up PR for the integration fix if that would be helpful.