fix(compilation): deep-clone Date/RegExp/TypedArray/Buffer/Error in compiled structuredClone; throw DataCloneError on uncloneable

[nickna]

Summary

Fixes #1255: the compiled/emitted structured-clone core ($Runtime.StructuredClone → StructuredCloneCore) was shallow — it only deep-cloned List<object?>/Dictionary<string,object?>/Dictionary<object,object?>/HashSet<object?> and returned everything else by reference, diverging from the interpreter's StructuredClone.Clone in two ways:

1. Cloneable-but-non-container types were aliased instead of deep-cloned — Date, RegExp, TypedArrays, ArrayBuffer, Buffer, Error instances.
2. Uncloneable values were silently passed through instead of throwing DataCloneError — functions/closures, symbols, class instances, Promise, etc., both at the top level and nested inside objects/arrays.

Fix

• StructuredCloneCore now deep-clones $TSDate, $RegExp, $TypedArray/$ArrayBuffer (independent copy, or a shared view when backed by a SharedArrayBuffer), $Buffer, $Object (recurses into its Fields dict), and the built-in $Error hierarchy (name-based reconstruction + Stack preservation).
• The previous alias-on-fallback is flipped to throw a new standalone $DataCloneError : Exception (no SharpTS.dll reference — stays fully standalone), matching the interpreter's exhaustive switch. This now correctly rejects uncloneable values at any nesting depth, not just the top level.
• A value whose CLR type lives outside the emitted assembly (e.g. an interpreter runtime value crossing in via CompiledMessagePortBridge, which already structured-clones on the interpreter side before handing off) passes through unchanged rather than throwing — only the compiled program's own uncloneable constructs throw.
• $MessagePort.PostMessage / $BroadcastChannel.PostMessage now catch $DataCloneError around the clone call and convert it to the existing receiver-side 'messageerror' sentinel, replacing the old top-level typeof pre-check (which missed nested uncloneables). $BroadcastChannel gained messageerror support end-to-end (sentinel + Drain handling), matching $MessagePort.
• WrapException recognizes $DataCloneError and returns its raw message string, matching the interpreter's catch binding for a non-guest-throw exception.

Bugs found and fixed along the way

• SharpTSBroadcastChannel.EnqueueMessageError's scheduled closure never invoked the property-style onmessageerror handler (only the EventEmitter event fired), and re-checked _closed at fire time — dropping an already-enqueued notification if the receiver closed before the timer ran, inconsistent with DrainPendingMessages' "in-flight deliveries complete after Close()" behavior.

Test plan

• dotnet test — 15452/15452 passing (22 new tests added)
• SharpTS.Test262 — 22/0 (4 skipped diagnostics), unchanged
• SharpTS.TypeScriptConformance — 31/0, unchanged
• Manual dual-mode verification of every example from the issue (Date/TypedArray/function/nested-function/symbol), plus RegExp/Buffer/Error/class-instance/plain-object, matching interpreter output exactly
• --compile --verify and --compile --standalone --verify both pass (no SharpTS.dll dependency introduced)
• New regression tests for $MessagePort/$BroadcastChannel messageerror (including a nested-uncloneable case) and structuredClone deep-clone/throw behavior in SharpTS.Tests/SharedTests/WorkerThreadsTests.cs and BroadcastChannelTests.cs