C API: implement context groups, global metadata, and syntax checking

Parents: #135, #134 Inspector dependency: #139

Complete the JSContextRef.h and JSBase.h functions not already implemented.

Acceptance:

• Implement JSContextGroup create/retain/release with exact shared-VM lifetime semantics.
• Implement JSGlobalContextCreateInGroup and JSContextGetGroup/JSGlobalContext.
• Implement global context name copy/set with correct string ownership.
• Implement JSCheckScriptSyntax with source URL, starting line, and exception diagnostics matching evaluation parsing.
• Implement JSGlobalContextIsInspectable/SetInspectable backed by real session behavior.
• Add C compile-link-runtime coverage, wrong-group/context checks, and teardown/leak tests.

Completion evidence

• a2524ae implemented global-context lookup, exact UTF-16 context names, and syntax-only parsing with evaluation-identical diagnostics.
• 5e2820d implemented shared-arena VM groups with distinct realms, same-group identity/exchange, context-retained lifetime, deterministic wrong-group rejection, and global JSClass lifecycle.
• 71bc743 implemented opt-in public inspectability backed by versioned concurrent runtime sessions and deterministic detach/teardown.
• Focused Zig coverage, real C/C++ hosts, and the system-JSC differential pass.
• The pinned macOS 27.0 public inventory is 117/117 complete with zero pending; the combined differential is 34/34 rows (a8ecd5e83f9f35ca).

Completed across a2524ae, 5e2820d, and 71bc743. Context groups, realm identity/lifetime, global metadata/class behavior, syntax checking, and inspectability are implemented with focused leak/teardown tests and system-JSC differential evidence.