ci: carry metrics baseline forward on no-code-change pushes (#3678)

* ci: carry metrics baseline forward on no-code-change pushes

The code-metrics job is gated on code_changed and only uploads the
metrics-tinyusb artifact on push, so a workflow/docs-only push to master (e.g.
removing an unrelated workflow) leaves the latest master Build run without a
baseline. PRs download the baseline from the latest master run, so the size
comparison then finds nothing and silently falls back to absolute sizes.

Add a small metrics-carry-forward job that, on a non-code-change push, downloads
the previous metrics-tinyusb artifact and re-publishes it, so the latest run
always carries a usable baseline. Carry-forward runs re-upload too, so the
baseline chains across consecutive no-code pushes (bounded by artifact retention).

0 files changed, 0 insertions, 0 deletions