Track D9Menu's position only while it is open

[PhilReinking]

D9Menu now renders its floating wrapper only while the menu is open. Closed menus no longer keep floating-ui's scroll and resize listeners, a ResizeObserver and an IntersectionObserver alive. Input's editor sidebar has one menu per block, so this grows with the form.

Fixes BITS-208

• MenuItems is static, so the items stay visible while the wrapper plays its leave transition.
• useFloating positions with left/top (transform: false), so the wrapper's slide-in translate isn't overridden.
• Props, slots, events and keyboard behaviour are unchanged. Input's four D9Menu call sites don't rely on the wrapper while closed, so Input needs no change.

Tested: check, build and tests pass; the new Menu spec fails on main. Headless Brave on Storybook: all five Menu stories, plus 20 menus in a scrolled box.

Not tested: Input with this build, and Chrome, Safari or Firefox by hand.

Before and after, 20 closed menus in a scrolled box (Storybook)

	main	this PR
Floating wrappers	20	0
ResizeObserver	20	0
IntersectionObserver	20	0
scroll listeners	60	0
resize listeners	60	0

• One menu open: 1 wrapper, 1 ResizeObserver, 1 IntersectionObserver, 3 scroll and 3 resize listeners. All back to 0 on close, right away; the wrapper still fades out for 100 ms.
• Same numbers with and without usePortal.
• Menu position matches main in every story: 10 px below the button, same edge, and it follows the button when the box scrolls.
• Enter: opacity 0, 0.24, 0.86, 1 over 200 ms. Leave: still fading at 50 ms, removed by 200 ms.
• Enter, Space and ArrowDown open with the first item active; Escape closes and puts focus back on the button.

Counted by wrapping ResizeObserver, IntersectionObserver and add/removeEventListener before page load, counting only calls that come from floating-ui.

🤖 Generated with Claude Code

[PhilReinking]

OK at b86f03a, CI green.

Checked beyond the PR text, in headless Chromium and WebKit: closed menus create no observers or listeners, the open position matches main in all four stories, and the menu neither flashes on open nor jumps while it fades out. Switching menus, an outside click and rapid toggling all end at zero. aria-controls exists only while open. The spec fails on main and when the v-if is removed.