Replace unsafe.Pointer for seen symbol tables with unique ID

[jakebailey]

We use reflect.ValueOf(m).UnsafePointer() in the checker in order to have maps as map keys themselves. This is the only place we use unsafe in the checker, and theoretically going through reflect has a high cost (see https://godbolt.org/z/3jMcxY49T).

Instead of doing this, we can instead note that we always get a symbol table from some other value that has an ID; a Node or a Symbol. If we use that ID (or a sentinel value) plus some metadata, we can avoid reflect and unsafe, swapping for a simple load+OR.

This seems to achieve the same goal. All tests pass.

[Copilot]

Pull request overview

This PR removes the last unsafe/reflect usage in the checker’s symbol accessibility logic by replacing map reference-identity checks for “seen” symbol tables with a compact, comparable symbolTableID derived from a NodeId/SymbolId plus a small kind tag.

Changes:

• Removed reflect.ValueOf(m).UnsafePointer() and unsafe.Pointer usage for symbol-table visited tracking.
• Introduced symbolTableID (kind-tag + source id) and threaded it through someSymbolTableInScope and related callbacks.
• Simplified global symbol table detection by passing an isGlobals flag instead of comparing map identities.