Add ZStack and lift the view builder child limit

[colemancda]

Stacked on top of #9 — review that first; this PR targets feature/modifiers and will retarget to master once #9 merges.

ZStack

ZStack had no Android renderer. It now renders as a FrameLayout, overlaying children in declaration order.

Alignment needs a little indirection: FrameLayout positions children through their own layout parameters rather than a container-wide gravity, and the renderer only has the parent's native view to work from when it mounts a child. The stack's gravity is therefore retained as the container's coordinator and read back when assigning each child's parameters. Only containers carrying a retained gravity get this treatment, so the frame layouts used for sheets and fragment hosting are untouched.

Children hug their content rather than filling the container, which is what gives the alignment room to act.

View builder child limit

@ViewBuilder was capped at ten children by a fixed set of arity-specific buildBlock overloads. Parameter packs replace all of them with one variadic overload, and TupleView's ten arity-specific initializers likewise collapse into a single variadic one. The empty and single-child buildBlock overloads stay as they are, so a lone child still passes through unwrapped.

The stacks gallery screen now declares thirteen children in one VStack, which previously did not compile.

Verification

On the emulator:

• ZStack renders correctly at .center, .topLeading, and .bottomTrailing.
• The thirteen-child stacks screen compiles and renders identically to the grouped version it replaced.
• Spot-checked Text, Navigation, Sheets, and List for regressions from the view builder change — all render and behave as before.