ci: gate the line length of new das code

[aleksisch]

Adds check_line_length, a CI gate on the width of new .das code, wired into run_extended_core (the per-PR extended-checks lane).

• Any added .das line over 100 columns fails.
• Any added comment over 70 columns fails, measured from its // / /* marker to the end of the line (indentation does not count; block-comment continuation lines are measured from their first non-blank column).
• Tabs count as 4 columns; a multi-byte UTF-8 character counts as 1.

Only lines the branch adds are measured: the diff runs from the merge-base with origin/<base-ref> (--base-ref, default master) to the working tree, limited to *.das. The comment scanner understands string literals, {...} interpolation holding strings, character literals, and nested /* */ comments. Files whose header says they are generated are skipped.

Locally: cmake --build build --target check_line_length.

This gate is temporary: it exists only until the whole codebase is moved to these limits; then the limits belong in a whole-tree check and this changed-lines gate goes away.

Files

• utils/internal/line-length/line_length.das - comment scanner, width measurement, diff parsing
• utils/internal/line-length/main.das - CLI (--base-ref, default master)
• utils/internal/line-length/test_line_length.das - 20 arms, registered in run_utils_tests

Testing

• test_line_length.das: 20/20 pass; negative controls (interpolation, nested block comments, quote character literal, block-continuation handling) each turn an arm red.
• Lint and das-fmt --verify clean on the new files; CMake configures and check_line_length sits under run_extended_core.
• The gate passes on this branch, and fails with main.das:66: comment is 83 columns, limit 70 when a long comment is appended.