fix(ui): isolate OpenTUI scroll reset compatibility

[atirna]

Problem

Shift+Wheel has to clear OpenTUI wheel remainders after Hunk reroutes the input horizontally. The previous inline private-member assertion used optional chaining, so an OpenTUI rename could silently let later input move the review viewport.

Approach

• isolate the OpenTUI 0.5.1 private access in one named compatibility bridge
• document the invariant and removal condition for a future public OpenTUI API
• throw a focused error if the required operation disappears instead of silently degrading
• cover the locked runtime operation and Shift+Wheel handling in both rendered and real-PTY tests

Validation

• bun run typecheck
• bun run lint
• bun run format:check
• bun run check:docs
• bun run test (2807 passed, 24 skipped)
• bun test src/ui/components/panes/DiffPane.test.tsx src/ui/AppHost.interactions.test.tsx --test-name-pattern "resetOpenTuiScrollAccumulators|shift plus (mouse wheel|native horizontal)" (5 passed)
• bun test test/pty/layout.test.ts --test-name-pattern "shifted mouse-wheel input" (1 passed)

Fixes #771

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