Horseshoe single-color paths show internal segment seams

## Problem

A horseshoe path with one effective color can still be rendered as multiple adjacent strokes. On rectangle paths this exposes internal seams, overlap, and round-cap artifacts, producing a chain of visible blocks instead of one continuous line.

## Required behavior

- `fixed`, `colorstop`, and `colorstopinterpolated` render one continuous state stroke.
- A one-color gradient renders one continuous stroke rather than adaptive gradient segments.
- Adjacent equal-color segments with no gap are combined visually.
- Internal boundaries always use `butt`; the configured line caps apply only to the complete visible start and end.
- Explicit gaps and genuinely different colors remain separate.

## Verification

Add unit and browser coverage for rectangle paths and verify the existing path shapes remain unchanged.