…16657)

## Summary
- Split `unwind_greenlets()` into two phases to reduce lock hold time on `greenlet_info_map_lock`
  - **Phase 1 (under lock):** snapshot greenlet IDs, names, frame pointers, and parent chains into lightweight `GreenletSnapshot` structs
  - **Phase 2 (lock released):** perform the expensive stack unwinding (`process_vm_readv` / `copy_type`) outside the lock
- Add `GreenletSnapshot` struct in `greenlets.h` to hold the snapshotted state

## Motivation
Before this fix, `unwind_greenlets()` held `greenlet_info_map_lock` for the entire duration of stack unwinding across **all** tracked greenlets. Since every greenlet switch calls `update_greenlet_frame()` under the same lock, applications with many tracked greenlets (e.g. gunicorn + gevent + psycopg2) experienced severe lock contention — greenlet switches stalled waiting for the sampler, leading to connection pool exhaustion and request timeouts.

### Why asyncio doesn't have this problem
Asyncio task unwinding (`unwind_tasks`) doesn't suffer from this because it reads task state directly from CPython internals (linked lists in 3.14+, or `WeakSet` in older versions) via `copy_type()` — no profiler-owned lock is involved during discovery or unwinding. The only profiler lock (`task_link_map_lock`) is held briefly for link-map cleanup, then released before the expensive coroutine stack unwinding begins.

Greenlets lack CPython-native tracking, so the profiler must maintain its own `greenlet_info_map` with frame pointers updated on every switch via `update_greenlet_frame()`. This creates a lock shared between the sampler thread (reading) and the application thread (writing on every switch) — the exact contention pattern this fix addresses by making the read-side (Phase 1 snapshot) fast.

### Regression test confirms the issue on main
The regression test (`test_gevent_greenlet_switch_not_blocked_by_profiler`) was cherry-picked to main in [#16660](#16660) and fails there, confirming this branch fixes the contention.

## JIRA
[SCP-1039](https://datadoghq.atlassian.net/browse/SCP-1039)

## Testing
- Added `test_gevent_greenlet_switch_not_blocked_by_profiler` regression test that:
  - Measures greenlet-switch wall time with 0 vs 2000 idle tracked greenlets (each with 50-deep stacks) while the profiler samples at 5ms intervals
  - Asserts the ratio stays below 3x to catch lock-contention regressions
  - Uses `gevent.joinall` with a 30s timeout and `try/finally` cleanup to prevent CI hangs

## Risks
- Snapshotted frame pointers may become stale between Phase 1 and Phase 2, but `unwind_frame()` already handles invalid pointers gracefully via `copy_type()` which returns non-zero on failure
- No change to public API

🤖 Generated with [Claude Code](https://claude.com/claude-code)

[SCP-1039]: https://datadoghq.atlassian.net/browse/SCP-1039?atlOrigin=eyJpIjoiNWRkNTljNzYxNjVmNDY3MDlhMDU5Y2ZhYzA5YTRkZjUiLCJwIjoiZ2l0aHViLWNvbS1KU1cifQ

Co-authored-by: taegyun.kim <[email protected]>