refactor(ios): extract the text-entry engine into RunnerTests+TextEntry.swift (#651)

Behavior-preserving relocation: move the text-entry concern out of the (formerly 1805-line)
RunnerTests+Interaction.swift — the repo's most-changed file — into a new
RunnerTests+TextEntry.swift. Interaction.swift drops to 1068 lines (-737).

Moved verbatim (no logic changes):
- value types: TextTypingRepairMode, TextEntryTiming, TextEntryResult, TextEntryTarget
- the focus -> type -> verify -> repair pipeline (typeTextReliably + focus orchestration +
  readiness polling + dropped-char/repair heuristics)
- clearTextInput and the text-entry leaf helpers (editableTextValue, isPlaceholderValue,
  isGenericTextInputLabel, normalizedElementText, moveCaretToEnd, estimatedDeleteCount,
  keyboardBecameVisible, keyboardElementExists)

The whole cluster is text-entry-exclusive (its only callers are within the moved code or the
already-internal command entry points), so every symbol keeps its original visibility — no
access widened. Shared helpers used by gestures/snapshot/get-text (isKeyboardVisible,
visibleKeyboardFrame, textInputAt, textInputCandidatesAt, readableText, ...) stay in
Interaction. The new file is auto-included via the project's file-system-synchronized group.

This is the safe, cosmetic half of the "TextEntry engine" architecture candidate. A deeper
extraction behind a real seam/type is intentionally deferred: it carries the #245 revert risk
and wants stronger text-entry e2e coverage first.

Verified: xcodebuild build-for-testing -> TEST BUILD SUCCEEDED. Pure relocation, no behavior change.

Author: thymikee