fix(chat): settle the send animation

[bmc08gt]

Four regressions in the send animation, all introduced alongside the chat edit work.

The bar's height change fought the send. A send collapses a multiline field, so setBarHeight runs at the same moment as the insertion and the settle-to-bottom. It used ChatMotion.swap (bounce 0.31) against a scroll spring set to 0.12 precisely so it wouldn't overshoot. It now uses keyboardScroll — zero bounce, the transcript following bottom chrome, which is the job it is doing.

The bottom inset was dropped rather than deferred. setBottomInset refuses to write mid-batch-update, but it discarded the request instead of holding it, so a bar height landing inside the update left the transcript matching it a layout pass later, as a snap. It now parks in pendingBottomInset and the batch update's completion applies it.

A send rebuilt the whole screen. ComposerModel.clear() wrote mode = .new unconditionally and @Observable fires on assignment without comparing. The chat screen's body reads isEditing and editingStableID, both derived from mode, so every send re-ran updateUIViewController at the frame the insertion started. Guarded to write only on a real change.

The bar's leading control tracked the field's top edge. Top-aligning the bar's row made it travel with every line the draft gained or lost, unanimated — the bar's springs key on chatExists and isEditing, neither of which moves during a send. Back to bottom-aligned.

The row above an insertion slid left. ChatColumnCell pinned one horizontal edge and let the other float, so the column's width was its widest arranged subview — for a short message, the receipt line rather than the bubble. Clearing the receipt shrank the column by 40pt and slid its origin the same distance; the bubble only looked still because its offset inside the stack moved the opposite way by exactly as much. That cancellation holds on final values, but mid-flight it is three separate layer animations that have to share a curve.

Both edges are pinned now and the stack's alignment hugs the sender's side. Measured at a 402pt row, the stack is (12, 0, 378, h) with and without a receipt, the own bubble's absolute x is unchanged at 347.67, other-sender rows still hug leading at 12, and long text still wraps at the cap. Only the receipt row animates. A full-width column also makes UIKit's default compressed horizontal measurement report the content's width, so the cell now measures height explicitly at the width the layout asked for — a width ChatLayout forces to full for a .fullWidth row anyway.