Fix fast Windows source scroll blanks

[theincrediblewwz]

What changed

• pin @codemirror/view to 6.43.8, which contains upstream tile-tree corruption fixes
• keep small sessions and sessions containing a very long Markdown block in normal block flow while retaining CodeMirror line virtualization
• keep TanStack block virtualization for ordinary large multi-block sessions
• replace the slow release-only scrollbar check with physical-speed CDP drags and in-drag gap, frame, long-task, runtime-error, endpoint-latency, and byte-integrity gates

Why

The previous regression test yielded to the renderer after every pointer step and only checked settled geometry. A real fast native scrollbar drag could therefore corrupt CodeMirror's tile tree, leave a large blank viewport, and still pass the test.

Validation

• Windows full suite: 494/494 passed
• focused public snapshot tests: 18/18 passed
• long-block regression: 12/12 top and bottom touches, no blank gap, endpoint p95 13.2 ms
• heterogeneous 80-block regression: 12/12 top and bottom touches, no consecutive blank frame, endpoint p95 39.4 ms
• production build, packaged real-session-copy probe, portable smoke, public-source gate, CI configuration gate, and byte-for-byte sanitized snapshot comparison passed
• after the first CI run exposed a disk-vs-React lock-state race in the Electron smoke, the check now waits for the rendered block lock state; the complete Electron smoke then passed 3/3 locally

The original user notebook was only tested through a temporary read-only copy and is not included in this repository or PR.