Stretch CSS Grid auto maximum rows

[wieslawsoltes]

Problem

A definite-height multi-row grid kept minmax(min-content, auto) at its intrinsic height instead of giving it the remaining block-axis space from the default align-content: normal behavior. In Code OSS Welcome this placed the footer near y=526 instead of Chromium's y=888.

Result

• preserves whether native grid tracks have an auto maximum
• distributes remaining height across all auto-maximum rows, including implicit rows and minmax(..., auto)
• excludes fixed, percentage, max-content, and fractional tracks from auto-maximum stretch
• retains explicit non-stretch align-content behavior through stylesheet cascade, inline override/removal, CSSOM, and layout invalidation
• adds matching portable parse, measure, and arrange behavior, including explicit fractional-row precedence and safe fallback for unsupported templates
• adds a required Chrome/native contract for exact geometry, multiple rows with gaps, max-content exclusion, cascade restoration, synchronous height mutation, and 500 publication mutations

The focused packaged Code OSS AppScene build now places the Welcome footer at y=888, matching Chromium exactly while preserving the named-area columns from PR #105.

Validation

• portable CSS tests: 358/358 passed
• focused native grid-auto-max-stretch: passed
• full native engine suite: passed, including existing performance gates
• Chrome contract: 1/1 document, 8/8 subtests passed
• native required contract: 1/1 document, 8/8 subtests passed
• 500 height/style mutations with forced geometry publication: under the 2000ms bound in Chrome and native
• packaged AppScene/WebScene Welcome+Chat geometry: browser footer y=888; native footer y=888
• git diff --check and capability/profile JSON validation: passed

Closes #106