fix(core): resolve css inherit on non-inherited properties to the parent value - #11521

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What is the current behavior?

Every css-wide keyword goes through one reset branch (isResetValue). InheritedCssProperty (color, font-size, ...) resolves the keywords per spec, but a plain CssProperty and a CssAnimationProperty map every keyword to the default. For a non-inherited property, initial, unset and revert → initial value is correct, but inherit must take the parent's value:

StackLayout { background-color: red; margin-top: 8; }
Button { background-color: inherit; margin-top: inherit; } /* default, not red / 8 */

Shorthands are worse: their converters only parse values, so margin: inherit, border-color: inherit, padding: inherit, background: inherit, ... fail to expand and are not applied at all, and font: inherit / transform: inherit turn into fixed values (font-style: normal, translateX: 0, ...).

What is the new behavior?

A stylesheet inherit on a non-inherited property resolves to the parent view's current value, or the default when there is no parent or the parent has no value.

• CssProperty: the css setter resolves inherit against the parent style and marks the value ValueSource.Inherited (also while the parent is at its default, so it keeps following it). _isCssValueStillApplied treats a recorded inherit as applied only while that mark is there, so a released local value makes the cascade write it again, while an untouched one is still skipped.
• CssAnimationProperty (background-color, opacity, width, height, transforms): the css slot holds the parent's computed value. It keeps following the parent under a local or keyframe value, so it is current when they are released.
• Live tracking: non-inherited values are never pushed down the tree, so a child that resolves inherit subscribes to the parent style's <property>Change event for that one property and writes inherit again on each change. The subscription is created only when a stylesheet resolves a property to inherit, and dropped as soon as the stylesheet (or, for a CssProperty, a local value) stops doing so. The common path adds no listeners, allocations or child walks: the parent's setter already checks hasListeners before notifying, and the setters only gain a source comparison.
  • Ordering: the parent's own css or a later local set reaches the child through the same event, whichever runs first.
  • Re-parenting: clearInheritedProperties (removal) and propagateInheritableCssProperties (add) re-resolve the followed properties, which moves the subscription to the new parent, or resolves to the default with none.
  • Native updates go through the existing setter paths unchanged (suspended updates, defaultValueKey capture/restore, valueChanged, equalityComparer, change events). A child following a parent at its default takes the reset path, so its own native default is restored.
• Shorthands: _expandCssShorthand expands a css-wide keyword into the same keyword on every longhand (margin: inherit → margin-top: inherit, ...), so each longhand applies it with its own semantics. This covers stylesheet rules, inline styles and var() substitutions that resolve to a keyword.
• Keywords set from code are unchanged and documented. view.style.backgroundColor = 'inherit' (or 'initial', ...) still just releases the local value. Unlike an inline CSS declaration it is not a sticky value that outranks the stylesheet; that is intentional and now documented on isResetValue and the CssProperty local setter.

Tests:

• packages/core/ui/core/properties/css-inherit-non-inherited.spec.ts (26 tests, 15 of them fail on main): parent value, following later parent changes, the parent's css applied after the child, chained inherit, no parent / parent at default, initial/unset/revert still → default, re-parenting (incl. dropping the old parent's listener), the stylesheet changing away from inherit, local value over inherit and release, the unchanged-inherit skip on the next pass, native writes, no listener on the parent when no child uses inherit, background-color/opacity, and margin/border-color/font: inherit plus margin: initial.
• apps/automated/src/ui/styling/css-keywords-tests.ts: added test_css_inherit_on_non_inherited_property and test_css_inherit_on_shorthand. test_value_after_inherit (code-set 'inherit' releases the local value) still holds, as it pins the intended local-keyword behavior; its stale TODO now says so. I reviewed the other automated style tests that use keywords or unsetValue (value-source-tests.ts, style-tests.ts); they exercise inherited properties or local resets and are not affected. The automated app was not run on a device or simulator for this PR.

Related: #11520 (local reset restores the cascaded value) and #11413 (inherited children reset cascade). This branch merges cleanly with each of them and with both together, and the core suite passes on the three-way merge. One interaction to settle once both land: #11520's cascadedCssValue treats a cascaded inherit as "no css value", so a local reset over a stylesheet inherit on a CssProperty falls back to the default until the next cascade pass, which then re-resolves it (a CssAnimationProperty is already correct). Routing a cascaded inherit through style[cssName] = 'inherit' in that path would close it.