Support RegExp buffer boundary assertions

Summary

Implement the Stage 3 TC39 proposal: https://github.com/tc39/proposal-regexp-buffer-boundaries

Implement their exact start, absolute-end, and before-final-line-terminator semantics in proposal-defined Unicode modes.

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

The custom RegExp compiler supports anchors, lookaround, Unicode modes, and modifiers but not \A, \z, or \Z.

Expected behavior

Implement their exact start, absolute-end, and before-final-line-terminator semantics in proposal-defined Unicode modes.

Scope notes

Extend RegExp Compiler, Program, VM, and runtime with correct literal/constructor grammar, backtracking, CRLF, sticky/global, and lastIndex tests. Preserve non-Unicode behavior. Additive syntax: no engine flag.

Related bytecode roadmap: #871 and #875. Update relevant documentation and the Goccia proposal inventory. Use the current proposal 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 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 current TC39 RegExp buffer-boundaries proposal, then trace the existing RegExp Compiler, Program, VM, and runtime seams. Review related bytecode issues #871 and #875 before defining the parser and execution changes. Done means focused project tests, pinned Test262/proposal coverage, documentation, and relevant suites pass without regressions.