fix: react to dynamic optional slots

[Banegasn]

Linked issue

Fixes #22

Summary

• Keep optional icon, supporting text, action, and navigation slots mounted in button, chip, list item, snackbar, and top app bar components.
• Use native slotchange events and hidden wrappers so dynamic additions, removals, and reassignment update layout without MutationObserver instances or manually managed listeners.
• Add browser suites for button and chip and dynamic-slot coverage for every affected family.

Acceptance criteria

• Optional slotted content added after connection appears immediately.
• Removing content collapses spacing immediately through hidden wrappers.
• Slot reassignment updates the correct region.
• Reconnect coverage verifies dynamic slot changes continue to work; implementation has no observer or external listener lifecycle to leak.
• Initial absent/present and dynamic transitions are covered for button, chip, list item, snackbar, and top app bar.

Risk and compatibility

• Risk: low. Existing slots and public APIs are unchanged; only their dynamic layout behavior is corrected.
• Semver impact: patch.
• Rollback: revert this commit.

Verification

Passed with pnpm 11.12.0:

• pnpm lint
• pnpm typecheck
• pnpm tokens:check
• pnpm build
• pnpm smoke:packages
• pnpm smoke:svelte-vite
• pnpm verify:package-licenses
• pnpm audit:prod
• Targeted and repository browser suites in Chromium, Firefox, and WebKit; affected totals: button 9, chip 9, list 51, snackbar 39, top app bar 33.

The repository-wide pnpm test command exercised all affected suites but its unrelated Angular app test fails under the local Node 26 runtime because localStorage is unavailable. CI pins Node 24.18.0; no Angular code was changed.

Documentation and accessibility

No public API or semantic change. Existing accessibility tests continue to pass; hidden wrappers remove empty regions from layout.

Release notes

Optional slotted button, chip, list item, snackbar, and top app bar content now updates immediately when it changes at runtime.