This is a pure refactor only change.

Replace the flattened transcript line metadata from Option<(usize, usize)> to an explicit
TranscriptLineMeta::{CellLine { cell_index, line_in_cell }, Spacer} enum.

This makes spacer rows unambiguous, removes “tuple semantics” from call sites, and keeps the
scroll anchoring model clearer and aligned with the viewport/history design notes.

Changes:

• Introduce TranscriptLineMeta and update TranscriptScroll helpers to consume it.
• Update App::build_transcript_lines and downstream consumers (scrolling, row classification, ANSI rendering).
• Refresh scrolling module docs to describe anchors + spacer semantics in context.
• Add tests and docs about the behavior

Tests:

• just fmt
• cargo test -p codex-tui2 tui::scrolling

Manual testing:

• Scroll the inline transcript with mouse wheel + PgUp/PgDn/Home/End, then resize the terminal while staying scrolled up; verify the same anchored content stays in view and you don’t jump to bottom unexpectedly.
• Create a gap case (multiple non-continuation cells) and scroll so a blank spacer row is at/near the top; verify scrolling doesn’t get stuck on spacers and still anchors to nearby real lines.
• Start a selection while the assistant is streaming; verify the view stops auto-following, the selection stays on the intended content, and subsequent scrolling still behaves normally.
• Exit the TUI and confirm scrollback rendering still styles user rows as blocks (background padding) and non-user rows as expected.