Open standard library error rows

[rtfeldman]

Standard library functions currently return a mix of open and closed concrete error rows, which prevents independent failures such as Str.split_first and U16.from_str from composing through ?. This makes concrete Try return rows throughout Builtin.roc extensible, including internal low-level helpers so they specialize directly without retagging wrappers, and adds regression coverage for the reported composition failure.

This keeps the existing closed-row language semantics unchanged; unlike the experiment in #10366, the change is confined to the standard-library API and its implementation contracts.

Closes #10490.

[greptile-apps]

Greptile Summary

The PR opens concrete standard-library Try error rows so independent failures can compose through ?, without changing closed-row language semantics.

• Updates concrete error annotations across JSON, string, iterator, list, and numeric APIs and their internal helpers.
• Adds a checker regression test composing Str.split_first with U16.from_str.
• Updates the affected REPL diagnostic snapshot.

Confidence Score: 5/5

The PR appears safe to merge, with no concrete correctness, compatibility, or security failures identified.

The changes consistently open composable concrete error rows, preserve intentionally closed internal contracts, and add direct regression coverage for the reported type-checking failure.

Important Files Changed

Filename	Overview
src/build/roc/Builtin.roc	Opens concrete Try error rows across standard-library APIs and corresponding helpers while retaining intentionally closed sentinel and capability contracts.
src/check/test/repros_test.zig	Adds focused regression coverage proving two independent standard-library failures can compose through ? without type errors.
test/snapshots/repl/json_nominal_scalar_codec.md	Updates the expected encode_f64 diagnostic type to display its newly open error row.

Flowchart

%%{init: {'theme': 'neutral'}}%%
flowchart LR
    A["Str.split_first(...)"] -->|NotFound| C["Open composed error row"]
    B["U16.from_str(...)"] -->|BadNumStr| C
    C --> D["Propagation through ?"]
    D --> E["Caller specializes the resulting Try row"]

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