Fix offset mapping for multi-character placeholders in SkikoParagraph

[adamglin0 (adamglin0)]

SkikoParagraphBuilder never adds the characters covered by a placeholder to the Skia paragraph; Skia represents each placeholder as a single U+FFFC. When an inline content item uses an alternate text longer than one character (e.g. appendInlineContent(id, "[icon]")), every Skia offset after it is shifted by length - 1 relative to the Compose text. SkikoParagraph passed Compose offsets to Skia unconverted, so hit testing, cursor rects, selection paths, line start/end and word boundaries after an inline item were wrong on all Skiko platforms, and a link placed after an inline item could not be clicked.

This change introduces PlaceholderOffsetMapping, built once per paragraph from the placeholder ranges (null when every placeholder is one code unit, so the common case only pays a null check), and routes every offset that crosses the Compose/Skia boundary in SkikoParagraph through it:

• Compose → Skia at the API boundary: getPathForRange, getCursorRect, getLineForOffset, getHorizontalPosition, getBidiRunDirection, getBoundingBox, getWordBoundary.
• Skia → Compose on outputs: getOffsetForPosition, getLineStart, getLineEnd, getWordBoundary.
• Internal Skia-space loops (getBoxForwardByOffset / getBoxBackwardByOffset, trailing-whitespace handling) are bounded by the Skia text length and read characters through the mapping.

Semantics: a Compose offset inside a placeholder range maps to the placeholder itself (caret before it, like Android's ReplacementSpan); a hit inside the placeholder resolves to its start or end depending on which half was hit, matching Android. The builder's single-U+FFFC contract and the public API are unchanged.

Fixes https://youtrack.jetbrains.com/issue/CMP-9009

Testing

• DesktopParagraphPlaceholderOffsetTest (ui-text desktopTest, 12 tests): getOffsetForPosition, getCursorRect, getBoundingBox, getHorizontalPosition, getPathForRange, getWordBoundary, line APIs, accumulated deltas with two placeholders, the single-character identity path, mixed single- and multi-character placeholders, a multi-line paragraph and an RTL paragraph. All positions are derived from the fixed-advance test font and placeholderRects, not from the APIs under test.
• BasicTextInlineContentOffsetTest (foundation skikoTest, 2 tests): the reported scenario, clicking a link placed after an inline item with a multi-character alternate text.
• With the mapping stubbed out, 11 of the original 13 new tests fail; only the identity-path test passes.
• :compose:ui:ui-text:desktopTest and :compose:foundation:foundation:desktopTest --tests "androidx.compose.foundation.text.*" pass.

Release Notes

Fixes - Multiple Platforms

• Fixed wrong hit testing, cursor and selection positions after inline content whose alternate text is longer than one character (e.g. links after an inline element could not be clicked). (CMP-9009)

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