node:v8: snapshot and profiler output parity cut #3296

Summary

Create the medium PR boundary for package-visible V8 diagnostic output surfaces that can return Perry-shaped data while preserving Node's observable stream/file/report contracts.

Child Issues

• node:v8: implement heap snapshot output #3140 - implement heap snapshot output
• node:v8: implement GCProfiler report shape #3142 - implement GCProfiler report shape

Intended PR Boundary

Implement the diagnostic-output layer after the basic node:v8 namespace exists:

• v8.getHeapSnapshot([options]) returns a readable stream that emits heap-snapshot-shaped JSON.
• v8.writeHeapSnapshot([filename[, options]]) writes a non-empty heap snapshot file and returns the written path.
• new v8.GCProfiler() exposes start, stop, and disposal behavior with a Node-shaped report object.
• Add or extend a V8 parity fixture to cover stream/file/report shapes without requiring exact V8 heap internals.

This cut is coherent because all children are diagnostic outputs where Perry-specific internal data is acceptable if the Node-observable shape, stream/file lifecycle, and error behavior match.

Node Behavior

Node v25.9.0 returns a readable HeapSnapshotStream from getHeapSnapshot(), writes .heapsnapshot files through writeHeapSnapshot(), and exposes GCProfiler instances whose start() returns undefined and stop() returns a report with keys such as version, startTime, statistics, and endTime.

Perry Behavior

docs/runtime-parity.md and docs/runtime-parity-gaps.md list the V8 snapshot/profiler APIs. Source search found no public node:v8 heap snapshot or GCProfiler implementation, and this checkout has no dedicated V8 parity fixture.

Duplicate searches run before filing:

• node:v8 heap snapshot GCProfiler profiler output diagnostics parity cut
• node:v8 diagnostics serialization heap statistics snapshot promiseHooks startupSnapshot GCProfiler parity cut

Non-goals

• Top-level v8.serialize / deserialize or Serializer/Deserializer classes.
• Heap-stat object helpers already covered by node:v8: diagnostics and serialization parity cut #2492 / node:v8: expose heap statistics shape #3138.
• Promise hook callbacks or startup snapshot helper state.
• Exact V8 heap/profiler internals beyond package-visible output shape.
• CPU profile and heap profile handle APIs unless they are filed as separate focused leaves.

Acceptance

• Focused parity/regression coverage proves child issues node:v8: implement heap snapshot output #3140 and node:v8: implement GCProfiler report shape #3142.
• Heap snapshot stream/file output is readable/non-empty and heap-snapshot-shaped for covered cases.
• GCProfiler constructor/method/report shape matches Node for covered cases.
• Related docs/gap metadata are updated for APIs covered by this cut.