Refit a background image when its bloom-canvas is sized after page load

[hatton]

Problem

Bloom fits a background image to its bloom-canvas once, during the page-load pass. When a container is laid out with JavaScript after that pass, the bloom-canvas inside it still has no size at fitting time, and three things go wrong:

1. The fit is computed from a box with no area. The numbers it writes are not neutral: they become the baseline that later resizes scale from, so the picture ends up somewhere arbitrary.
2. Nothing can ask for a re-fit once the container has its real size.
3. A bloom-canvas nested inside another bloom-canvas loses its flower placeholder, because the two rules that hide the placeholder on a background image use descendant combinators, so the outer canvas's elements satisfy the :has() of the inner one.

What the PR does

• adjustBackgroundImageSizeToFit returns early when the bloom-canvas has a width or height of zero or less (a hidden canvas with a border reports a negative size), leaving the fitting to whoever knows the real size. A hidden or not-yet-laid-out canvas is in that state.
• CanvasElementManager gains two public methods: adjustAfterContainerResize() re-fits every bloom-canvas on the page, and refitBackgroundImage(bloomCanvas) re-fits one and returns a promise that settles when the image is loaded and fitted. The general resize path preserves and scales each child's offsets, which is right for elements the user placed and wrong for a background image fitted before layout.
• Every place that looks up a bloom-canvas's background image goes through one function, getBackgroundCanvasElement, which takes the first background image whose nearest bloom-canvas is the one asked about. A plain descendant search would hand an outer canvas the inner canvas's picture; a direct-child search would miss the Image Description tool's temporary wrapper.
• The two placeholder rules in placeHolderImages.less use child combinators. A canvas element is always a direct child of its bloom-canvas, except while the Image Description tool wraps it in a bloom-describedImage, and that case hides the placeholder anyway.
• A new test file covers the zero-area guard (zero and negative sizes, plus a sized canvas that is still fitted) and the scoped lookup (nested canvases with and without their own background image, and the Image Description wrapper).

Notes for the reviewer: this PR targets master on purpose, as groundwork that should land on master alone; the temporary Version6.5 rule in AGENTS.md does not apply to it. The two new methods have no caller in this PR. They are the seam that a later feature (custom JavaScript layouts, which size a container after the page-load pass) will call.

Devin review

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This change is 

[hatton]

[Claude Fable 5.1 from Hatton's machine during preflight] Consulted Devin on 2026-09-04 21:15 UTC up to commit be753fdb0d179b4dcf1f7f4060796661ad56637d. Two passes ran. The first raised two bugs (nested-canvas background lookup, negative-size guard) and two Investigate flags (promise not returned, test coverage of the new public methods); all four are mirrored as review threads above, each with its outcome, and Devin's re-review marks both bugs fixed. The re-review added one flag about the PR's base branch, answered on its thread: master is the developer's deliberate choice. No informational items. CI passed.