[dajiaohuang]

Related: #128, #93

Changes

• Preserve the Eyes in QR code get slanted/mangled (elliptic arcs are rendered incorrectly?) #128 arc correction: the previous dx+dy check could discard a valid arc with distinct endpoints when the deltas summed to zero. Coincident-endpoint arcs remain omitted, and zero-radius arcs remain lines.
• applyViewbox now reads the parser-normalized preserveaspectratio key and matches the standard case-sensitive YMid and YMax alignment tokens. This fixes the existing Symbol/use viewport path; it does not initialize root Document or nested Group viewports, so this PR does not resolve all of viewbox is not correctly mapped to SVG viewport #93.
• Reverts the unsupported fixed-length path-grouping change; no rendered grouping defect is claimed.

Validation

• PHP 8.3.6 with DOM/XML: local base-vs-candidate regressions through Document and a recording SurfaceInterface showed the Eyes in QR code get slanted/mangled (elliptic arcs are rendered incorrectly?) #128 arc Bézier count 0→1, YMid translate-y 0→50, preserveAspectRatio="none" scale-y 1→2, and xMaxYMax translate-y 0→100.
• These are parser/surface-call results, not PDF or visual acceptance. The full PHPUnit suite and candidate CI were not run; prior green checks belong to the previous head. The normal push may trigger the configured PHP 7.1–8.4 workflow; no manual CI action was taken.