[christianparpart]

What

• Row marks: Canvas::mark(row, column, sequence) anchors a zero-width raw sequence (for example an OSC 133 zone mark) to a canvas cell. Marks live in the Buffer (addMark(), marks(), clearMarks()) and are cleared with the frame, like images.
• Emission rule: the inline screen writes a mark through writeRaw at its own column before repainting any cell of its segment, from the mark's column up to the next mark's on that row. Marks are diffed like hyperlinks: a row whose marks (column and sequence, in order) differ from the previous frame repaints whole; an otherwise unchanged row emits nothing. A mark on a wide character's continuation column is dropped, image rows emit no marks, and the fullscreen and fixed viewports ignore marks. When two marks share a column, the later one wins.
• Column-0 contract: terminals carry zone state across rows, so a component that needs per-cell correctness marks column 0 of every row it can repaint. This is documented on Canvas::mark.
• TerminalOutput::deleteLines(n) writes DL (CSI n M).
• MockTerminalOutput::rawWrites() / deletedLines(): the mock records every writeRaw with the cursor position it was made at, and every deleteLines.
• core::platform::readMachineId(FileSystem const&) (<core/platform/MachineId.hpp>): reads /etc/machine-id through an injected FileSystem, returns it lowercased only if it is exactly 32 hexadecimal digits, and std::nullopt for a missing file, uninitialized or other content. Not part of the WebAssembly subset.

Why

endo's shell integration places OSC 133 zone marks on exact prompt rows, which needs marks that survive partial repaints and a way to delete lines. It also needs the machine id for OSC 3008. Neither is endo-specific, so they belong here.

Consumer impact

All additions; no existing signature changes. TerminalOutput::deleteLines is a new virtual with a default implementation, so an existing TerminalOutput subclass keeps compiling.

• contour (vendored): none
• endo: moves its pin to use row marks, deleteLines and readMachineId
• fastcached: none
• tuidu: none
• Lightweight dbtool: none
• morph (including its WebAssembly build): none; readMachineId is outside the WebAssembly subset

Tests

New cases: Screen.rowMarks_* (first frame emits at the column, unchanged row emits nothing, segment re-established on repaint, left-segment repaint, reservation scroll, full redraw, changed sequence or column over identical cells re-emits, image row, continuation-column mark), TerminalOutput.deleteLines_writesDL, and MachineId.* (valid, missing, uninitialized, wrong length, non-hex digits).

Run locally on the rebased branch: clang-debug build and ctest (76/76), gcc-release build and ctest (76/76, canary cases skipped as designed), clang-tidy preset build clean, clang-format.py --all --check, mkdocs build --strict.

• CHANGELOG entry under [Unreleased] (or not user-visible)
• Documentation updated (or nothing it describes changed)