refactor(compile): extract per-receiver branches from EmitGetProperty (#1242)

[nickna]

Closes #1242. Part of #1094.

What

Follow-up to #1141 / PR #1241, which split RuntimeEmitter.Objects.Properties.cs at the file level but deliberately left the 2,028-line EmitGetProperty god-method intact. This PR extracts the per-receiver branches inside that method into Emit<Receiver>GetBranch(il, …) helpers, the original #1141 ask.

15 self-contained, disjoint receiver arms move into a new sibling partial Compilation/RuntimeEmitter.Objects.GetPropertyBranches.cs. Each helper mirrors the already-factored EmitProxyGetPropertyCheck/EmitProxyDeleteCheck shape:

• emits the receiver-type test itself,
• runs the branch body + ret on a match,
• otherwise falls through to a caller-supplied notMatch label.

The dispatch table in EmitGetProperty now reads as a flat sequence of EmitXGetBranch(il, …, notX); MarkLabel(notX) arms sitting right beside the existing proxy-check call.

Extracted arms: namespace, $Object, Map, Set, Dictionary, $Array, List, object[], String, $Buffer, $Stats, $TSFunction, $BoundTSFunction, $CJSModule, $RegExp.

EmitGetProperty drops from ~2,028 → ~960 lines.

What was left inline (deferred, per the incremental plan)

The genuinely interdependent arms #1242 flagged as risky stay inline:

• the System.Type / class-instance pair (they share the base-walk-loop jump target classInstanceLabel),
• the three-way Promise dispatch (Task / $Promise subclass / $Promise),
• the typed-array family ($ArrayBuffer/$SharedArrayBuffer/$DataView/TypedArray share a gated block),
• the bound-callable multi-check ($BoundArrayMethod/$BoundMapMethod/$BoundSetMethod/$BoundAnyFunction),
• the primitive-prototype fall-throughs (bool/double).

These remain candidates for later increments.

Safety

Pure mechanical move — no behaviour change. The larger arms ($Object, $RegExp, Dictionary) were relocated with a line-preserving script rather than retyped, so their local functions and comments carry over verbatim. Each arm was extracted incrementally, one receiver type at a time, and verified with --compile … --verify (IL validation) plus an output diff against a pre-refactor compiled baseline; every increment matched.

No new metadata-token references are introduced — the moved IL keeps its existing Ldtoken/GetMethodFromHandle late-binding, so compiled standalone DLLs gain no SharpTS.dll dependency.

Gate

• dotnet test (xUnit): 15404 / 0 failed / 0 skipped
• Test262 (interpreter + compiled): 22 / 0 / 4 skipped — no baseline regression
• TypeScriptConformance: 31 / 0
• IL --verify: clean on the emitted GetProperty