feat: completion-order streaming (Branch 1 — kill head-of-line delay)

[rsanheim]

Branch 1 of 2 in the "make fetch feel fast" divide-and-conquer. This one evolves the sticky-footer printer; the sibling branch (rjs/git-all-tty-ratatui) explores a full ratatui TUI.

Stacked on the Phase 1 branch (rjs/git-all-ui-feedback-c07c9b, PR #31) so the diff here is just the completion-order delta: one file (printer.rs) + the doc.

Problem

The trace on git-all fetch over 104 repos exposed the real cause of "feels slow": the printer buffered finished rows behind the alphabetically-earliest unfinished repo.

• first_exit_ms=2432 — first fetch finishes at 2.4s (network floor)
• first_print_ms=9794 — but the first row didn't appear until 9.8s
• max_ordered_wait_ms=10637 — worst repo waited 10.6s to display after finishing

So repos were completing but the screen looked stuck for ~7s. (No wall-clock regression — status benched identical to main, 3.85s vs 3.87s.)

Change

TtyTablePrinter now streams each repo's row the moment it finishes, tracking printed rows by index instead of a contiguous cursor. The live scrollback is in completion order; the pinned footer (counts, clock, in-flight list) is unchanged.

Non-TTY / redirected output is untouched — PlainPrinter keeps deterministic alphabetical order, so pipes and scripts stay stable.

Results (~/work status under a PTY)

metric	before	after
first_print_ms	771 (status) / 9794 (fetch)	225 (= first_exit)
delayed_repos	98	11
max_ordered_wait_ms	1592	1
total_ms	3641	3641

Verified

cargo test green (updated the buffering test to assert completion order), clippy/fmt clean. Driven under tmux with script/tty-lab — mid-run, a slow early repo (activities) is still in the footer's running list while six later-alphabetical repos have already streamed to scrollback. Clean rendering, no scrollback garble.

[cache-layer  ] clean          <- finished first, shown first
[agentic-dev  ] clean
[billing-svc  ] 1 untracked
[amion-api    ] clean
[backend-core ] clean
[data-pipeline] clean
----------------------------------------------------------------
SUMMARY  6 of 24 done | 6 running | 12 pending | 2.5s
running: activities, dashboard, edge-proxy, gateway, growth-tools, identity