docs: add e-ink refresh-latency curve and boot-time budget notes

docs/notes/boot-time-budget.md

@@ -0,0 +1,73 @@
+# Boot-time budget — where the ~4.3 s to cursor goes
+
+> **Measured 2026-07-11:** cold boot is **4258 ms** power-on → cursor (the
+> `boot: cursor ready` log prefix). That clears the **≤ 5 s v0.1 gate** with
+> ~742 ms to spare. This note breaks the number down, and argues that the
+> **≤ 3 s v1.0 target** is hard on this panel because one ~1.9 s full refresh is
+> architecturally unavoidable at cold boot.
+>
+> Notes index: [`README.md`](README.md). Docs index:
+> [`../README.md`](../README.md). Backs the boot-time acceptance criterion in
+> [`../v0.1-mvp-product.md`](../v0.1-mvp-product.md#acceptance-criteria) and the
+> v1.0 goal in [`../roadmap.md`](../roadmap.md). Refresh cost model:
+> [`../tradeoff-curves/epd-refresh-latency.md`](../tradeoff-curves/epd-refresh-latency.md).
+
+## The waterfall
+
+From the boot serial log (power-on → first editor frame + input loop live):
+
+| Phase | ~ms | Lever |
+| --- | ---: | --- |
+| ROM + 2nd-stage bootloader + app image load | ~550 | flash speed (DIO now; QIO / 80 MHz ≈ −200 ms, speculative) |
+| PSRAM init + **memtest** + heap | ~920 | `CONFIG_SPIRAM_MEMTEST=n` → **−730 ms** (kept **on**: a real HW sanity check on a hand-wired board) |
+| EPD reset + init | ~130 | fixed panel bring-up |
+| **Splash full refresh** | ~1850 | e-ink floor — see below |
+| SD mount + note load | ~70 | quick on the genuine 32 GB SDHC |
+| USB host install + git thread spawn | ~60 | background |
+| **First editor render** (full-area partial) | ~680 | already fixed from ~1870 ms (was a *second* full refresh) |
+| **Total** | **~4260** | |
+
+Two lines carry the weight: the **splash full refresh (~1.85 s)** and the
+**first editor render (~0.68 s)**. Everything else is ≤ ~0.9 s combined, and the
+biggest of *those* — the ~0.73 s PSRAM memtest — is a deliberate keep.
+
+## The insight: one full refresh is unavoidable, so the splash is nearly free
+
+After power-on the panel controller's `0x26` "previous" RAM bank holds garbage. A
+partial refresh *diffs the new image against that bank*
+([`../tradeoff-curves/epd-refresh-latency.md`](../tradeoff-curves/epd-refresh-latency.md)),
+so the **first clean paint must be a full refresh** (~1.9 s) to establish a known
+image. There is no way around this on this panel short of a different waveform.
+
+That reframes two things:
+
+- **The splash costs almost nothing.** Boot needs one full refresh regardless; the
+  splash simply *is* that refresh, turned into a "boot is happening" affordance.
+  Dropping the splash would **not** save the 1.9 s — the editor's first frame would
+  then have to be the full refresh instead. (This is exactly what the old boot did
+  and why it paid *two* full refreshes.)
+- **The v0.1 win was removing the second full refresh, not the first.** Once the
+  splash has seeded a clean baseline, the editor rides in on a full-area *partial*
+  (~0.63 s) instead of a second full refresh (~1.9 s) — the ~1.25 s saving that
+  took cold boot from ~5.5 s to ~4.26 s. Verified clean on-panel (no splash
+  ghost behind the editor text).
+
+## Is ≤ 3 s (v1.0) reachable?
+
+To go from ~4.26 s to ≤ 3 s needs ~1.26 s cut. The honest lever list:
+
+- **PSRAM memtest off:** −0.73 s → ~3.5 s. Costs the boot-time hardware check;
+  reasonable once the board is no longer hand-wired.
+- **Faster flash boot (QIO / 80 MHz):** ~−0.2 s, speculative, needs a bench check.
+- **Overlap cheap init under the splash busy-wait:** SD mount + note load + USB
+  install (~0.13 s total) currently run *after* the splash refresh returns, but
+  the refresh is a `wait_while_busy` spin — those could be kicked off before it.
+  Saves ~0.1 s at most.
+
+Even stacked, that lands around **~3.2 s** — still over. The ~1.9 s full-refresh
+floor is the wall, and it can't be cut without dropping the clean first image or
+moving to a faster panel/waveform. **Conclusion:** ≤ 3 s is marginal-to-unreachable
+on the GDEY0579T93 as driven today. When v1.0 comes, either revisit the target
+(≤ 3.5 s is achievable with the memtest off), or accept the splash as the
+deliberate cover for the one refresh e-ink makes us pay. Recorded here so the v1.0
+boot-time item is scoped against physics, not optimism.