fix(ui): stop the install progress bar from garbling non-scroll-region terminals

[fullstackjam]

What does this PR do?

Removes the scroll-region "sticky" progress bar; the install log now renders top-to-bottom on every terminal.

Why?

The install progress bar (StickyProgress) reserved the bottom rows as a frozen status bar using a DECSTBM scroll region (\x1b[1;Nr). On terminals that report a normal TERM but don't actually honour scroll regions, every newline scrolls the whole screen, so the reserved divider+status bar got dragged into the log and reprinted on every 80ms tick — the install read as repeated, overprinted garbage instead of clean top-to-bottom output.

It was most visible during npm (its per-package installs finish fast, so the bar redraws rapidly), but the code is shared with brew, so both were affected. IsScrollRegionSupported() only checked TTY + TERM + terminal height — it never verified real scroll-region support, so it couldn't tell a supporting terminal from a non-supporting one.

The fix drops the scroll-region path entirely and always draws a plain in-place \r\033[K status line that trails the output. Each package prints on its own line, in order, with one live spinner line at the bottom — on every terminal. This deletes scrollregion.go and its whole failure mode (net −262 lines).

Testing

• go vet ./... passes
• Relevant tests added or updated (removed the now-obsolete scroll-region tests; kept a Start/Finish smoke test)
• Reproduced the corruption and verified the fix by driving the real StickyProgress code through a PTY and replaying the byte stream in a terminal emulator (pyte):
  • On a scroll-region-ignoring terminal: dividers rained through the output before the fix; clean top-to-bottom after.
  • On a compliant terminal: clean before and after (no regression), at 24×40, 40×100, and with long wrapping error lines.
• Full make test-unit (L1, incl. archtest) green; e2e/vm-tagged build compiles.

Cross-repo checklist

• Does this need a docs/content update in openboot.dev? — No.
• Does this change the CLI ↔ server API contract? — No.

Notes for reviewer

• Docs updated to match: docs/HARNESS.md "What's intentionally NOT in the harness" now records why the scroll region was removed and says not to reintroduce it; internal/ui/AGENTS.md drops the scrollregion.go row.
• Out of scope, flagged separately: npmStepDone counts npm failures as "installed" in the summary line (bar.Increment() instead of IncrementWithStatus(ok), unlike brew). That's a counting bug, not this rendering one, so it's left for a follow-up.