Capturing a tuple in a runtime expression throws at hydration

[AmeinEskinder]

expr.md lists "Tuples of vocabulary types" in the shared vocabulary, and expr_field.rs:21 compiles pair.0 into JavaScript pair[0], so a tuple is meant to be an array on the browser side. The Rust side serializes it as one. hydrateSurrogate has no array case, so every expression that captures a tuple throws in the browser.

Reproduction

$(pair.0) with let pair = (1.5f64, 2.5f64) renders:

<!-- ::topcoat::expr::start("(() => { const [__external0] = [cx.hydrate([1.5,2.5])]; return __external0[0]; })()") -->1.5<!-- ::topcoat::expr::end -->

The server text is correct. The JavaScript is not. cx.hydrate([1.5,2.5]) reaches hydrateSurrogate with an array, typeof value === "object" while value.t is undefined, so it falls through to the default arm:

Error: Unknown surrogate type: undefined
 ❯ hydrateSurrogate src/surrogate/index.ts:73:12

$(pair) on its own does the same, with return __external0 in place of the index.

Where it comes from

• crates/topcoat-runtime/src/lib.rs:52 writes a captured value into the page as cx.hydrate(<json>).
• A tuple surrogate is a plain Rust tuple of surrogates, so serde emits an array: (1.5f64, 2.5f64) gives [1.5,2.5], and (1.0f64, Some(2.0f64)) gives [1.0,{"t":"Option","v":2.0}].
• browser/src/surrogate/index.ts has no array member in DehydratedSurrogate and no array case in hydrateSurrogate.

Reach

scan() walks the document in a single for loop and there is no try/catch in scan.ts, text.ts or binding.ts, so the throw leaves the loop and nothing after the offending expression is hydrated. I have verified the throw itself; I have not measured the page-level effect in a browser.

One thing a fix should not miss

Hydrating the array into a plain array of surrogates is what the generated pair[0] needs, but the text path would then disagree rather than throw. NodeViewParts for (T1, T2) concatenates its elements with no separator, so the server renders $(pair) as 1.52.5, while String(array) at text.ts:55 would give 1.5,2.5. Attribute values have the same shape: AttributeValueViewParts for (T1, T2) concatenates, and is present when any element is present.