headless ant/term ↗

test: tie bce's terminfo declaration directly to its actual behavior The declared-vs-omitted capability lists in this file are two independently hand-maintained facts about the same code -- exactly how bce drifted stale last time: this file asserted bce was *absent* with a comment claiming erase ops always fill with COLOR_DEFAULT, which quietly went wrong the moment blank_cell(g) started filling with the active background instead, and nothing caught it until it corrupted a real nvim session. Adds a test that performs an actual erase with an active background, checks whether the result behaves bce-style, and asserts the declaration matches that behavior rather than checking them as separate claims. Verified it fails when the two are put out of sync (temporarily undeclaring bce against unchanged behavior) and passes once they agree again.