docs(sync): merge the commit handoff into the staging tradeoff curve

The bench phase the handoff was written for is closed (splice benched,
fast-seek landed, cache removed, decision made), so the note's live half —
the firmware plumbing plan — moves into sync-commit-staging.md as "The fix —
wiring the O(depth) splice into the firmware", reconciled to the run-5 state.
The duplicated TL;DR/measurement summaries are dropped in favor of the
curve's own trail; inbound references (notes index, git_bench comments)
now point at the curve.

docs/tradeoff-curves/README.md

@@ -11,4 +11,4 @@
 | --- | --- |
 | [`wifi-auto-sync.md`](wifi-auto-sync.md) | `auto_sync` interval vs Wi-Fi energy (a `1/T` hyperbola) — why the default is 10 min and opportunistic, not a wall-clock timer. |
 | [`epd-refresh-latency.md`](epd-refresh-latency.md) | E-ink refresh latency vs rows driven — the full / full-area-partial / windowed-Y cost model behind typing responsiveness and the boot splash→editor swap. |
-| [`sync-commit-staging.md`](sync-commit-staging.md) | Commit-staging strategy vs working-tree size — `add_all(["*"])` (O(tree) FAT walk) vs explicit-path (O(churn)); the walk-vs-writes split that decides whether explicit staging is worth it. |
+| [`sync-commit-staging.md`](sync-commit-staging.md) | Commit-staging strategy vs working-tree size — RESOLVED: every index-based path is O(N_tree) and fails on the real repo (611 s / OOM); the O(depth) TreeBuilder splice is benched at ~2–2.8 s and ships. Holds the full measurement trail plus the firmware plumbing plan (merged from the retired handoff note). |