Module render 

Where a component renders — the Mode A / Mode B decision, and what it costs to be wrong.

docs/05-tier-lowering.md §5.1 gives two rendering modes over one source:

	Mode A — thin	Mode B — local
view runs on	server	client
Wire carries	DOM patches	data patches (state diffs)
Optimistic UI	no	yes — the same fold runs locally, reconciled by seq

The row that decides everything is the second one. Mode A sends the browser a rendering of the state; Mode B sends it the state. Everything below follows from taking that literally.

The rule: a Mode B page may not be a function of who is asking

A view has the shape (state, session) -> Html. If it reads the session, it renders a different page for different actors from the same state — which is to say it is filtering, scoping or hiding by identity. Running that view on the client requires giving the client the state it filters, so every actor receives what the filter was removing. The page would still look right. That is the worst kind of wrong.

So a component whose view is per-session is refused Mode B, and the refusal names the reason rather than a rule. What is left is exactly the class §5.1 and docs/10-decisions.md D5 describe as Mode B’s: pages that are the same function of the same state for everybody — editors, typeaheads, drag-and-drop, anything single-user or public.

This is a placement rule and not a lint: it is decided from the slicer’s own account of the view (crate::split::Roles::view_is_per_session), which is the same fact §3.8’s fanout analysis reads, so a program cannot be per-session for one of them and not the other.

Optimism is a property of what crosses, not of a component

“The browser applies the expected event to its local copy speculatively — legitimate because it runs the same pure fold the server runs” (D5). That is only available to a client that holds the value the fold is of. A client holding a projection — a session’s filtered list, say — could not apply an event to it without a second, different fold that no program writes. So optimism is not an extra feature layered on Mode B; it is the same fact stated twice, and this module reports it as one decision with two consequences.

Structs

Decision

How one component renders, and what follows from it.

Enums

Mode

Where a component’s view runs.

NoOptimism

Why a Mode B client may not guess.

Why

Why a component renders where it does.