api: POST /api/scenario returns result plus graph topology (M3 slice 1)

One panel's whole world in one call: effective config in, echoed
config + cascade result + undirected edge list out. Edges are
[from, to] pairs with from < to in deterministic node order;
Graph.Edges() walks the adjacency once, GraphEdges(config) rebuilds
the seeded world (~25us) so Result stays lean and /api/comparison
stays untouched. This closes the topology gap the design brief
flagged; the frontend's seeded d3-force layout consumes these pairs.
Go bits: [][2]int is a slice of fixed-size arrays; [2]int is a value
type, comparable, and JSON-marshals to [a, b], exactly the wire
shape the spec asks for.
tygo regen includes a fix: engine.Strategy now maps to the generated
Strategy type instead of decaying to 'any' in ScenarioRequest.
Verified live through the dev stack: 7/120 reached, 351 edges.

docs/m3-implementation-handoff.md

@@ -0,0 +1,81 @@
+# Handoff: milestone 3 implementation (from the UI design session)
+
+You wrote `docs/ui-design-brief.md` and asked for a design session. It ran on
+2026-06-10, every decision made with Justin via AskUserQuestion, and it is
+done: nothing UI-shaped is left open. This doc tells you what exists now,
+what changed relative to your milestone 3 assumptions, and where to start.
+
+## What exists now
+
+- `docs/ui-spec.md` is the normative spec. Page layout with wireframe,
+  component tree, every interaction with trigger/behavior/states, API
+  requirements, visual tokens for both themes, a testable accessibility
+  checklist, 12 ordered implementation slices, and one-line rejected
+  alternatives. Build from it without re-deciding; if you disagree with
+  something, raise it with Justin, do not silently deviate.
+- `docs/mockup/index.html` is the approved visual reference (v2; Justin
+  rejected v1 outright). It is a design artifact with stand-in layout and
+  edges, not app code. To view: bake a live `/api/comparison` response into
+  it (replace `__DATA__`, see `data.json` + `rendered.html` already there),
+  open in a browser, `?dark` for the dark theme, narrow window for mobile.
+  Match its look; do not port its code.
+
+## Deltas against what you probably assumed for milestone 3
+
+- The dashboard is not the fixed three-strategy comparison. Panels are a
+  dynamic list (cap 6) of scenarios: one shared base `Config` plus sparse
+  per-panel overrides and a strategy. The classic trio is just the default,
+  loaded from a study preset module (`web/src/presets/deepfake-school.ts`)
+  that owns all study copy, so no research question is hardcoded.
+- New endpoint needed, and it is slice 1, your Go territory:
+  `POST /api/scenario` takes `{config, strategy}`, returns
+  `{config, result, edges}` where `edges` is the deterministic topology as
+  node-index pairs. This closes the topology gap flagged in the brief. The
+  engine already has the graph; expose it. Same validation and 400-string
+  behavior as comparison. tygo regen, determinism test on edges.
+  `/api/comparison` stays as-is (parity table and external consumers).
+- The frontend fires one `/api/scenario` per panel in parallel, debounced
+  400 ms, atomic swap, keep-last-good on errors. No batch endpoint
+  (rejected, one-liner in the spec).
+- The optimisation feature (your milestone 5) got its API shape reserved:
+  `POST /api/optimize` goal-seek, request/response sketched in spec
+  section 2. M3 ships no optimizer UI, only a layout that can host it.
+- In scope for M3 beyond what the brief listed: export (JSON, per-node CSV,
+  PNG snapshot), both light and dark themes with a toggle, and mobile as a
+  first-class target (Justin expects first contact by phone). Animated
+  GIF/WebM export is explicitly post-M3.
+- The accessibility criteria your locked decisions called UNDEFINED are now
+  defined: spec section 8 is the checklist, and slice 12 is the audit.
+- Pinia and Vue Router: decided not needed. Module-scope composable store,
+  modal focus, one page.
+
+## Calls Justin made explicitly (do not relitigate)
+
+- Focus view is a modal overlay (recommendation was expand-in-place).
+- Sharing is address-bar-only, no Share button, no toast
+  (`history.replaceState` after every successful run).
+- Both themes ship, not light-only.
+- Dynamic panels, export, and the optimizer hook all came from him
+  unprompted; they are wanted, not gold-plating.
+
+## Method notes
+
+- Your working agreements stand: teaching-paced TDD on Go (slice 1 gets a
+  micro-brief, failing test first, small annotated diffs, learning-log
+  commit message); Vue/TS driven fast and mostly autonomously.
+- Justin's design bar is high. His words on the first mockup: "a very weak
+  mishmash of elements"; he wants a clean, responsive SPA that "looks
+  professional and feels amazing". The approved mockup is the bar: app
+  shell, type scale, one card system, real SVG icons, consistent spacing.
+  Never show him boxes-on-a-page, not even as an intermediate state demo.
+- Dev stack runs in the shared tmux session "spreadlab" (`./dev.sh`, API on
+  :8080, Vite on :5173). Use it, do not spawn background tasks.
+- One shared percent formatter everywhere; the 82-vs-83 rounding mismatch
+  already bit the mockup once (spec section 6).
+
+## First steps
+
+Read `docs/ui-spec.md` end to end, view the mockup in both themes and at
+phone width, then propose the slice 1 plan (the `/api/scenario` endpoint)
+and start the usual loop. Spec section 10 is the full M3 slice order; each
+slice leaves the app shippable.