docs: add optimization-decisions analysis, update README future plans

Detail the shared-encode and torch.compile tradeoffs with data-driven
estimates from the pipeline code. Shared encode saves ~1.2-2.3s per
variant (2-4% total) and is deferred; torch.compile needs a 30-minute
ROCm spike before deciding. Update Current State and Future Improvements
to reflect the analysis.

README.md

@@ -586,15 +586,22 @@ Use `--raw` to bypass weighting and fall back to plain diffusers encoding.
 | `vnasset upscale` | ✅ Working |
 | Self-contained torch wheel | ✅ Working |
 | `vnasset serve` (daemon/HTTP API) | 🚧 Planned |
-| `torch.compile` on UNet | 🚧 Planned |
-| Batch edit loop (shared VAE encode) | 🚧 Planned |
+| `torch.compile` on UNet | 🔬 Needs spike (see [decisions](docs/optimization-decisions.md)) |
+| Shared encode optimization | 📋 Analyzed, deferred (see [decisions](docs/optimization-decisions.md)) |
 
 ## Future Improvements
 
-- **`torch.compile` on UNet** — the UNet forward is identical each step; ROCm's
-  `torch.compile` support is maturing and could cut per-step time significantly.
-- **Shared encode optimization** — for N edit variants of the same input image,
-  run VAE encode and VL visual token encoding once, then only text-encode and
-  denoise per variant.
 - **`vnasset serve`** — lightweight daemon with Unix socket or HTTP API for
   integrating VNAsset into external tools.
+- **`torch.compile` on UNet** — the UNet forward is identical each step, but
+  ROCm `torch.compile` maturity is uncertain. Needs a 30-minute spike to measure
+  actual speedup and check for graph breaks / crashes on RDNA 3.5. If the spike
+  shows 15%+ and stable, implement with a warmup step at session load.
+- **Shared encode optimization** — for N edit variants of the same input image,
+  saves ~1.2–2.3s per variant (VAE encode + vision encoder). The dominant cost
+  (transformer denoising, ~45–50s) cannot be shared, so the overall gain is
+  2–4%. Deferred until workload shape demands it (100+ variants). When built,
+  the right place is the pipeline runner, not the session API.
+
+See [`docs/optimization-decisions.md`](docs/optimization-decisions.md) for
+full tradeoff analysis with data and cost estimates.