feat(extensions): cap pane size responsively

[benvinegar]

Problem

Extension panes can use a responsive preferred fraction, but their maximum size is fixed in terminal cells. A pane that should stay subordinate to the review can therefore consume too much space on smaller terminals or after a resize.

Approach

• add ExtensionPaneSize.maxFraction in extension API v26
• cap both planned geometry and divider dragging against the host body axis
• apply the tighter of max, maxFraction, and review minimum-space constraints
• retain capped manual overrides across later terminal growth
• document and package-check the public contract

This is deliberately an extension pane sizing primitive, not a new built-in pane policy.

Verification

• bun test packages/hunk/src/extensions/runExtension.test.ts packages/hunk/src/ui/lib/extensionPanes.test.ts packages/hunk/src/ui/hooks/useExtensionPaneController.test.tsx (87 passed)
• bun test test/pty/extensions-integration.test.ts (21 passed)
• bun run typecheck
• bun run lint
• bun run format:check
• bun run check:docs
• bun run deps:check
• bun run build:npm && bun run check:pack
• bun run changeset:status
• bun run test (2,198 passed, 3 skipped, 1 unrelated failure in packages/hunk/src/lib/shikiWasm.test.ts; reproduced unchanged on origin/main)

Evidence

No visual capture yet; this is opened as a draft while deciding whether to keep the API.

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Project	Deployment	Actions	Updated
hunk-web	Ready	Preview	Sep 10, 2026 1:22pm UTC