Fix: Column honours fill_height: true (iOS)

[C-Sinclair]

Summary

The .column case in MobNodeView (ios/MobRootView.swift) only set
maxWidth on its frame. A Column with fill_height: true therefore
collapsed to the natural height of its children — only Box was honouring
node.fillHeight. This broke the canonical full-screen pattern:

<Column fill_width fill_height>
  <Header />
  <Scroll>...</Scroll>   # expected to flex
  <Footer />             # expected to pin to bottom
</Column>

On iOS the footer sat directly beneath the last child instead of the parent's
bottom edge.

Change

ios/MobRootView.swift, .column case:

- .frame(maxWidth: .infinity, alignment: .leading)
+ .frame(maxWidth: .infinity,
+        maxHeight: node.fillHeight ? .infinity : nil,
+        alignment: .topLeading)

• node.fillHeight is already parsed by mob_nif.m (same prop Box uses);
  no NIF or renderer changes required.
• Alignment switched from .leading to .topLeading so children anchor at
  the top when the column flexes. When maxHeight is nil the VStack still
  hugs its content vertically, so the default path is unchanged.

Backward compatibility

Pure addition. Default fill_height is false, matching today's intrinsic-
height behaviour. No call sites need updating.

Out of scope

• Android — android/ in this repo is JNI/Zig only. Compose-side parity
  (if needed) would live in the downstream host/demo app.
• Swift tests — there is no SwiftUI test target in this repo; per
  CLAUDE.md native UI changes are verified by mix mob.deploy --native
  plus a screenshot or Mob.Test interaction. Adding a test target is
  out of scope here.

Testing

• Deploy a screen with <Column fill_width fill_height> containing a
  header, a flex region, and a footer inside a fixed-height parent;
  confirm the footer pins to the bottom.
• Confirm columns without fill_height still size to intrinsic height
  (no regression on existing screens).

[GenericJam]

Thanks @C-Sinclair — landed as 27c9442 in mob 0.6.15 with your authorship preserved. Six-line Swift fix for the Column fill_height: true symmetry bug. Closing since the commit is on master.