ref(seer): Replace PageContext with SeerContextProvider

[Mihir-Mavalankar]

Renames and rewrites the structured page context system for Seer Explorer. The original draft had three blocking issues that prevented it from working:

Nesting never worked — registerPageContext called ctx.registerNode(nodeType) without passing parentId, so all nodes landed at root level. Fix: a second React context (SeerNodeContext) carries the current component's nodeId downward so child HOC wrappers can read it as their parentId synchronously during render.

Initial data writes were silently dropped — useSeerContext(data) effects fire before the parent HOC's registerNode effect (children run before parents in React). With the reducer-based approach the UPDATE_NODE_DATA case was a no-op if the node didn't exist yet. Fix: data is now stored in an imperative useRef<Map> and written synchronously without dispatch. getSnapshot() reads structure from the reducer state and data from the ref at call time.

Infinite re-render loop (OOM) in tests — the context value object was recreated on every useReducer dispatch, giving the HOC's useEffect a new ctx dep on every registration, triggering cleanup+setup in a loop. Fix: useMemo on the context value so callbacks are stable.

Other changes:

• registerSeerContext(nodeType, Component) — two args, no options/extract bag
• useSeerContext(data) write overload and useSeerContext() read overload returning { getSeerContext(componentOnly?) }
• Flat node map with parentId pointers; tree assembled lazily at getSnapshot() time
• 7 integration tests covering nesting, unmount cleanup, data updates, full-tree and subtree reads, and no-provider graceful render

[azulus]

I don't think we need the third prop here per previous conversations. I'd mostly use the register function just to wrap + name and make the third prop optional (we might want to use it for filtering data later or some such)

[azulus]

I think this would be better served using the hook escape hatch (usePageContext({ title: 'Error Rate'});) or whatever it should be called to dedupe the "context" overloading

[azulus]

there should probably be a test in here which mounts, renders, then confirms that the PageContextProvider is actually providing the page context for sending to the AI. I'd also add a test with a nested case so we can test that DummyChart in DummyWidget in DummyDashboard (or something similar) nests appropriately and all are providing the right data in the right structure