Second pass over the page memory model (done by Codex/gpt-6-astra in an
isolated worktree, reviewed and merged here):
- Resize/reflow now port Zig's incremental ReflowCursor algorithm with
page backfill and the same capacity growth/split sequence, instead of
rebuilding a snapshot, so page boundaries, pruning and tracked-pin
remapping match Ghostty.
- Point<->pin conversion for the active area starts at the list tail and
the viewport offset is cached, so hot-path operations no longer walk
the whole scrollback from the head.
- RefCountedSet reproduces Zig's observable ID reuse and error points.
- All 202 PageList tests are ported (plus 13 extra regressions).
With this, the five Screen/Terminal tests previously ignored as blocked
on PageList behaviour pass and are re-enabled.
Test Plan:
- cargo test -p ghostty-terminal (913 passed, 0 ignored)
- cargo clippy -p ghostty-terminal --all-targets -- -D warnings
ddidderr
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