fix(virtual-list): report only ranges the list actually draws

[guyverino]

Problem

MoonVirtualList::on_visible_range invoked its observer from the closure handed to uniform_list. That closure is also used to MEASURE one row: measure_item renders item_to_measure_index..+1 from request_layout and again from prepaint, before the real range exists. The observer therefore saw a phantom 0..1 twice per frame ahead of every real range.

An observer that evicts state for rows outside the reported range — keyboard focus, an open popup — then evicted everything but the measured row on every frame. Downstream, MoonTerminal's Connections tab blurred the core-name field one frame after the click, so a core could not be renamed at all. The other consumer, the chart stack, only survived because its handler is an idempotent assignment that the real range repaired later in the same frame.

Change

UniformList gains its own on_visible_range channel, invoked once per prepaint with the range that goes to the item renderer (flipped indices included, while decorations keep receiving unflipped positions). MoonVirtualList forwards to it; its public signature is unchanged.

Two halves to the reporting rule, and both are needed:

• a list holding no rows reports 0..0;
• a list that holds rows but shows none of them — zero height, mid-collapse, not yet sized — stays silent, so a squeezed list is never mistaken for an empty one.

An emptiness test alone reports 12..13 for a collapsed list whose offset is not a whole multiple of a row; a height test alone silences a merely padded list whose row is plainly visible, because Taffy floors the element at its own padding.

The builder now documents what the runtime does not promise: reporting is per prepaint rather than per frame (a cached ancestor skips it, a retried Window::transact pass repeats it), cx.notify and Window::refresh are dropped mid-draw while Window::blur still takes effect, and a scroll requested from the observer lands on the next frame.

Verification

• tools\run-component-guardrails.ps1 → PASS (437 component tests, 7 gallery, audit/API/mirror baselines unmoved).
• cargo test -p moon-gpui → 180 passed. Not part of CI, run locally because this touches the runtime.
• Six new gpui::test cases plus one in moon-gpui, held by a new audit contract virtual_list.visible_range_reporting. Each was proven by mutation: wiring the observer back into the renderer yields [0..1, 0..1, 0..10]; reporting visible_range instead of the rendered range makes a flipped list claim 40..50; the old padded-box guard yields [] instead of [11..12]; dropping the height half yields [12..13] for a collapsed list; deleting a test drops the contract to Fail.
• MoonTerminal built against this branch through a local [patch], cargo test --workspace green, and the Connections rename confirmed working by hand.

docs/MOON_PATCH_QUEUE.md records the patch so a re-sync of the GPUI extraction restores both halves of the rule.