[P1] Make conditional slot layouts reactive to dynamic content

[Banegasn]

Objective

Ensure slotted icons, actions, supporting text, and navigation controls added after first render appear and lay out correctly.

Evidence

Button, chip, list-item, snackbar, and top-app-bar call light-DOM queries during render and omit slot wrappers when content is absent. Adding/removing slotted content does not itself request a Lit update, and an omitted slot cannot emit slotchange to recover.

Scope

Always render slots with CSS-driven presence where possible, or introduce a small reusable slot-presence controller. Cover add/remove/reassign and cleanup across affected packages.

Acceptance criteria

• Optional slotted content added after connection appears immediately.
• Removing content collapses spacing immediately.
• Slot reassignment updates the correct region.
• No MutationObserver or listener leak occurs after reconnects.
• Tests cover initial absent/present and dynamic transitions for each affected component family.

Dependencies

Blocked by #5 and #6.

Autonomous execution contract

Prefer native slot/CSS behavior; introduce observation only where the platform cannot express the layout.