test: direct-mode shadow-buffer invariant over forward/reverse/forward

[willwade]

Summary

• Regression guard for Direct Mode (RFC 0015): replaying the output callback's deltas (0=insert, 1=delete-as-backspaces-per-code-point, 2=clear) into a shadow buffer must always reproduce dasher_get_output_text() byte-for-byte
• 20 scenarios: shallow (≈one word) through deep reversals, with and without pointer-up pauses

Investigation context

This test was written while investigating a Dasher-Windows build .24 report ("zoom back to delete a word or a bunch of words, zoom forward again — it messes up just the previous word"). The invariant holds on both v0.1.6 (the pin .24 shipped with) and current main — the engine's insert/delete stream is consistent. The corruption was the Windows newline-injection path (fixed in Dasher-Windows bfdafb4: KEYEVENTF_UNICODE 0x0A is ignored by target apps; now mapped to VK_RETURN), which made paragraph-node deletes eat a character of the preceding word.

Kept as a permanent guard: any future engine-side drift in the delete/re-emit path breaks direct mode on every frontend.

Greptile Summary

The PR adds a Direct Mode regression test that replays output callback events into a shadow buffer and checks that it remains byte-for-byte consistent with the engine’s output across forward/reverse/forward navigation.

• Registers the new test executable with CMake and CTest.
• Exercises 20 reversal-depth and pointer-pause scenarios.
• Validates insert, Unicode-aware delete, and clear callback events against polled output text.

Confidence Score: 5/5

The PR appears safe to merge.

No blocking failure remains.

Important Files Changed

Filename	Overview
CMakeLists.txt	Registers the Direct Mode shadow-buffer regression executable through the existing public C API test helper.
tests/test_direct_mode_shadow.cpp	Adds a callback-replay invariant test covering twenty forward/reverse/forward navigation scenarios.

Sequence Diagram

sequenceDiagram
    participant Test
    participant Engine as DasherCore
    participant Shadow as Shadow buffer
    Test->>Engine: Drive forward navigation
    Engine-->>Shadow: Insert callbacks
    Test->>Engine: Drive reverse navigation
    Engine-->>Shadow: Delete callbacks
    Test->>Engine: Drive forward again
    Engine-->>Shadow: Re-emitted insert callbacks
    Test->>Engine: dasher_get_output_text()
    Test->>Shadow: Compare byte-for-byte
    Shadow-->>Test: Matching invariant

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