Checkpoint: today's fixes before reconciling with the rust-rewrite worktree

Fixes a live-reproduced VT-switch busy loop (failed page_flip retried
with no backoff), shadow rendering bleeding onto occluding windows
unclipped, and a winit-backend buffer-age correctness bug that left
stale cross-window pixels on screen. Committing before merging in the
much larger uncommitted rust-rewrite worktree, which independently
touches several of the same files - this is the pre-merge baseline to
diff against, not a claim that these are the final versions of these
fixes.

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