Finish issue 10770: phantom nominal arguments and shared top-level constants

[rtfeldman]

Closes #10770. With this, the issue's own reproduction mounts: roc build --target=wasm32 --opt=size of repro/nightly-2026-08-13-generic-browser-signal/app.roc against roc-signals at c0e4a9c, mounted with scripts/browser/mount_wasm_example.mjs, prints mounted minimal, and the location-source example fixture passes its spec end to end.

Two remaining causes, both independent of #10814 and #10818.

Relating a call's request to the callee's checked signature descended into a nominal's backing and returned, never relating the nominal's own declaration arguments. Every argument the backing mentions is reached that way, so only phantom arguments were lost: a Capability(a) whose payload never mentions a left a unresolved in the callee's request, and an unresolved type variable finalizes as uninhabited, so the capability's closures specialized at an uninhabited type and lowered to a reachable runtime error. That is the hit a runtime error the issue reports, and it is why the failure tracked the payload type: Str, Bool, U64 and List(U64) worked while { path : Str }, (Str, Str) and [Visible, Hidden] did not. A declaration's arguments are components of the value the same way a record's fields are, so the traversal now relates them like every other constructor it walks.

Separately, a top-level binding is one value, so every reference must read the same allocation, and a binding whose value contains a callable got neither: the ConstStore's function values were ruled out of static data, so each reference materialized its own erased callable and a platform handed that binding twice received two pointers. Static data already emits an erased callable as one allocation naming its procedure through a relocation, so a capture-free function value is fully decided by the ConstStore and belongs there. Two conditions make that safe: a restoration reached while an enclosing ConstStore node is still being built reads that node's binding local, which a standalone static initializer procedure cannot name, so it no longer becomes static data; and a request that already names a committed type is sealed before it keys the static-data use, which is what lets two references to one constant share a single allocation. Without the first condition test/cli/issue_9889_roc_parser/Issue10303Sgf.roc fails ARC certification with an unbound local in a static initializer.

test/fx/phantom_capability_parameter.roc covers the first, driven through the io-spec matrix on every backend; it prints hit a runtime error without the change. It needs the fx platform's new store_seed!/take_seed! pair, whose result type is the only place its type variable appears, since nothing else lets a test build a phantom capability without constraining it from a call site. test/provided-callable-host gains a top-level binding read by both fields of a provided root's result for the second; those two fields arrive 0x40 apart without the change.