fix(Paragraph): Remove Firefox "inline-block" hack for <*-text-line>, use local DOMRect-cache.

[mettta]

Removed a legacy inline-block workaround for service text-line elements and kept a single inline display path.

Why the workaround was added:

• It was introduced to reduce Firefox issues with inline offsetTop measurements and baseline gap noise during line-splitting.

Why we remove it now:

• Firefox no longer shows the issue this workaround targeted.
• The workaround caused a mismatch between measurement-time layout and final render layout.
• In edge cases, text looked like it fit during calculation, but wrapped after grouping.
• This was more visible with custom fonts and mixed inline content.

Also, to stabilize measurements:

• Wrap every split line (not only compound, but also single text lines) in textGroup: each line gets a block-level wrapper, while inline flow is preserved inside the group, keeping the original visual appearance.
• Replace offsetTop/offsetHeight-based line-start detection with cached DOMRect geometry in _findNewLineStarts, preserving the original heuristic while fixing Firefox inline offset inconsistencies.