perf(ios): improve codeblock

[eszlamczyk]

What/Why?

Speeds up iOS code block rendering. Each fenced code block was being laid out from scratch multiple times per render: boundingRectWithSize for measurement and drawAtPoint for drawing each spin up a throwaway TextKit stack, lay out the whole block, then tear it down again on every drawRect: (scroll, redraw, streaming). tree-sitter parsing was never the bottleneck; the redundant layout was.

Two changes, both in ENRMCodeBlockContainerView.m:

1. Analytic height. Code never wraps and every line fragment is a fixed height (codeBlockLineHeight, or the font's natural line height), so the laid-out height is exactly lineCount * perLineHeight. This replaces measureHeightForCodeBlockNode: (height only) becomes essentially free.
2. Cached TextKit stack, paint-only draw. The content view builds and lays out its NSTextStorage/NSLayoutManager/NSTextContainer once, when the attributed code changes, then caches them. drawRect: only calls drawGlyphsForGlyphRange:atPoint: on the already-computed layout, clipped to the dirty rect so off-screen lines are skipped. Scroll/redraw/streaming no longer re-lay-out. Scroll content width falls out of usedRectForTextContainer for free, removing the separate width measurement pass. Same rendering engine drawAtPoint used internally, so output is pixel-identical.

Measured gains (iOS simulator; 4 code blocks ~15 lines each — js/python/ts/go - with text between, whole document remounted 10x; per-render = total phase time per full document render):

phase	baseline (ms)	after (ms)	change
parse	3.72	2.66	untouched (noise)
apply	0.20	0.17	untouched (noise)
measure	7.41	1.72	-77%
draw	7.06	1.31	-81%
measure+draw	14.47	3.03	-79%
total	18.40	5.87	-68%

Additionaly the perf win should be even greater while streaming

Testing

• Example app, code block screens: blocks render identically to before — same height, no vertical shift, long lines still scroll fully (checked the tab-indented Go block specifically, since width now comes from CoreText/TextKit layout rather than a full bounding-rect pass).
• Streaming: code blocks still render per-keystroke while open and pick up syntax highlighting on close.

PR Checklist

• Code compiles and runs on iOS
• Code compiles and runs on Android
• Updated documentation/README if applicable
• Ran example app to verify changes
• E2E tests are passing
• Required E2E tests have been added (if applicable)

[Copilot]

Pull request overview

This PR optimizes iOS fenced code block rendering by removing repeated full-text layout work during measurement and drawing, replacing it with (1) an analytic height calculation and (2) a cached TextKit layout that drawRect: only paints from.

Changes:

• Replaced bounding-rect based height measurement with an analytic lineCount * perLineHeight calculation.
• Added a cached NSTextStorage/NSLayoutManager/NSTextContainer stack in the code content view and updated drawing to paint only the dirty glyph range.
• Switched code-block content width calculation to read from TextKit’s usedRectForTextContainer.

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