Support do expressions

Summary

Implement the Stage 1 TC39 proposal: https://github.com/tc39/proposal-do-expressions

Parse and compile do expressions with proposal-correct completion values, lexical scope, precedence, and abrupt completion.

Why

This proposal fits GocciaScript's modern ECMAScript surface and the 0.13.0 proposal roadmap. It should extend the engine's existing architecture rather than arrive as a userland shim.

Current behavior

Blocks cannot currently produce a value in expression position.

Expected behavior

Parse and compile do expressions with proposal-correct completion values, lexical scope, precedence, and abrupt completion.

Scope notes

Target the bytecode-first architecture after #875, or preserve temporary parity if the tree walker remains. Cover empty/nested blocks, closures, and control-flow early errors. Previously-invalid additive syntax: no flag.

Related bytecode roadmap: #871 and #875. Update the relevant language/built-in documentation and Goccia proposal inventory. Use the proposal repository and its current specification at implementation time; add focused project tests and matching pinned Test262/proposal coverage without regressions.

Acceptance criteria

• Parser/API surface, descriptors, evaluation order, errors, and observable protocol behavior match the current proposal.
• Implementation uses the identified shared engine seam without a duplicate compatibility shim.
• Focused positive, negative, abrupt-completion, and edge-case tests cover the proposal.
• Relevant project suites, pinned Test262 cases, formatting, and documentation checks pass.

Start with the TC39 do-expressions proposal and the bytecode-first architecture referenced after #875; review related bytecode roadmap issues #871 and #875 before locating the shared engine seam. Done means proposal-correct parsing and compilation, focused positive, negative, abrupt-completion, and edge-case coverage, matching pinned Test262 or proposal tests, and updated documentation without regressions.