view: node ids are ::-joined strings that the page splits again #2061

Found in the review of #2057, which adds the experimental view. It does not block that PR; #2059 fixed the findings that did. Paths refer to #2059's head.

Problem

Node ids in the view model are flat strings with structure in them, which the CLAUDE.md type-safety rules ban.

• NodeBuilder::render_id in crates/graphcal-project/src/view_model/project.rs falls back to format!("{}::{}", module_leaf, name) for an imported declaration with no spelling.
• web/view/src/formula.ts recovers the parts with lastIndexOf("::").
• The fallback uses only the module's leaf name, so two modules with the same leaf (a/lib.gcl, b/lib.gcl) can produce the same id. Not reproduced. Since view: fix the must-fix findings of the #2057 review #2059, the projection rejects a duplicate id (DuplicateNodeId) instead of drawing wrong wires.
• The page builds composite string keys too: `${from}->${to}` and "\u0000"-joined keys in main.ts and render.ts, and the keys in selection.ts.

Proposed fix

• Make id an opaque key, and send the spelling a reader sees (e::p_budget) in its own field.
• On the page, replace the composite keys with nested maps or a type for an id pair.

This changes the wire format, so it belongs with the other model contract changes.