[simonhamp]

A long list can now send only the rows near the screen and placeholders for the rest, without anything on screen moving.

on-visible-range on <native:list> calls the method with the positions of the first and last rows on screen, each time either end has moved by visible-range-step rows (10 by default). Rows are counted the way end-reached counts them, with a section's rows in their place. It's registered with the virtual list's virtual_window callback kind and sends the same "first,last" payload, so the method gets two ints and core needs no change.

<native:list on-visible-range="showRows" :visible-range-step="20">

<native:list-placeholder native:key="..."> is an empty row drawn at the height the list last measured for the row with the same key. A node's id comes from its key and its parent's, so the placeholder takes the row's place in the list instead of being a new row. Lists measure their rows while they report their visible range. A row the list never measured takes estimated-height, or the average of the rows it has measured. Outside a list it draws nothing.

Why: the dev/null app's Log keeps every commit it loads. Scrolled to 1,520 commits on an iPhone 16 Pro, jetsam killed it at a 3.4 GB footprint. With this, the app draws the rows near the screen and placeholders for the rest (NativePHP/dev-null-app#51). Simulator, same canned API, #144 and NativePHP/mobile-air#561 in both. Before is main with #144:

commits	footprint before	after	PHP before	after	quiet render before	after
200	451 MB	421 MB	72.5 MB	72.5 MB	34 ms	28 ms
600	500 MB	452 MB	106.5 MB	80.5 MB	76 ms	34 ms
1,200	663 MB (at 1,440)	487 MB	154.5 MB	88.5 MB	213 ms (at 1,440)	33 ms
2,000		524 MB		104.5 MB		40 ms

The app holds 6,000 to 8,000 NativeUINodes from 200 to 2,000 commits. Frames stay small too: at 1,800 commits the app's frame is 5,813 nodes with placeholders and 61,741 without. On the phone, the frame without them reached 104,482 nodes at 1,800 commits and went over EDGE's 16 MB frame buffer. With #144 alone it held 37,679 at 1,200.

Things to know when reviewing:

• iOS: when an off-screen row's content changes, SwiftUI's List drops its size to its 52pt estimate and doesn't measure it again until it's displayed. That already happens to any off-screen row whose content changes (a relative time ticking over does it), and placeholders make it happen to a few dozen rows each time the app moves its window. UIKit moves the content offset for the rows above, so nothing on screen moves: screenshots before and after moving the window by 40 rows are pixel-identical, light and dark, and frame-by-frame motion while scrolling back up through those rows matches the same list with no placeholders. The content height and scroll indicator do change. Keeping sizes exact would need a UIKit list.
• iOS: every publish makes the rows on screen fire onDisappear and then onAppear, so the tracker reads the range 50ms after it moves (no more than every 150ms). Reporting straight away named a single row. It counts each row's views, because a row and its placeholder are different views and the new one can appear first.
• iOS: a very fast fling skips rows altogether, so they're never laid out or measured. Their placeholders take the average.
• A status-bar tap from deep in a long list animates through placeholders, so the list is blank for the moment it takes to reach the top.
• Android: LazyColumn keeps its place by the first visible item, so off-screen rows changing size don't move anything, and the height cache only matters for a placeholder that's on screen. Compiled in the dev/null app, not run on a device or emulator.
• Measuring is an onGeometryChange per row on iOS and an onSizeChanged box per row on Android, only while on-visible-range is set.
• The new row closures (measuring, on screen, off screen) follow Stop iOS list rows from keeping old copies of the whole list #144: static helpers build them from the row's id and the list's height cache or tracker, never the list or its nodes. Neither object holds nodes.
• Not part of this PR: most of what still grows natively is mobile-air's shadow thread autoreleasing parse objects into a pool it never drains. Wrapping each frame in autoreleasepool kept Swift strings flat (1.7k at 200 commits and 1.5k at 2,000, against 156k without it). That goes in its own mobile-air PR.

Tests: tests/ListTest.php covers the attributes, the callback kind, and a placeholder getting the id of the row whose key it carries.

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