Harden Unicode search and terminal graphics edge cases

[Marc-André Moreau (mamoreau-devolutions)]

Summary

• Preserve search selection by logical-line ID and cell offset through reflow, duplicates, scrollback eviction, and viewport changes; retain text-based fallback for Ghostty's reconstructed row IDs.
• Add the Unicode 16.0.0 GraphemeBreakTest fixture with license and LF checkout policy. Assert exact cells and chunk invariance for all 369 terminal-representable cases; classify the remaining 724 cases explicitly instead of claiming full pure-segmenter conformance.
• Add Sixel, OSC 1337, ConEmu, and Kitty boundary, malformed-input, placement, crop, scrollback, eviction, reset, and rendering regressions, with narrow production fixes for uncovered defects.
• Document measured parity and intentional unsupported behaviors.

Validation

• dotnet test Devolutions.Terminal.slnx --no-restore --verbosity quiet (exit 0; Core 1112, Control 142, Render 62 passed; existing Connection 9 and Browser E2E 2 skips)
• dotnet build Devolutions.Terminal.slnx --no-incremental --no-restore --verbosity quiet (0 warnings, 0 errors)
• dotnet format Devolutions.Terminal.slnx --no-restore --verify-no-changes --include ... for changed C# files (passed)
• git diff --check (passed)

The Core terminal-cell suite does not constitute full UAX #29 conformance for the excluded Unicode rows; see docs/parity-status.md for the categorized limits.