fix: restore function names for migrated async guards

[rocketman-code]

Current Behavior

When a Guard detects thread migration on drop, the migrated record is attributed to "<migrated>" instead of the actual function name. This is because the 16-byte Guard optimization (PR #112) removed the function name from the Guard struct to minimize timing overhead.

For async code with multiple migrating functions, all migrated invocations collapse into a single "<migrated>" bucket with no way to distinguish which function each record belongs to.

Expected Behavior

Migrated guards should report the actual function name so that per-function timing data is preserved even when tasks migrate across threads.

Possible Approaches

• Look up the function name from the enter thread's TLS stack using the cookie (stack depth) stored in the Guard
• Embed a function name index (interned string table) and store an index in the Guard instead of the full string
• Use a compact function ID assigned at registration time

The constraint is keeping Guard small (currently 16 bytes) to minimize timing distortion from stack allocation.

Related

• Accurate self-time for migrated async functions #94 tracks the separate problem of self-time overcounting for migrated functions. This issue is specifically about losing the function name identity.

Scope

• piano-runtime/src/collector.rs