greenlet 3.0.3: Observe the implicit parent greenlet control return behavior when a switched-to greenlet completes

## Search Result
`search_known_solution` for `greenlet.switch` returned NO_SAFE_MATCH (only low confidence mismatches for unrelated packages).

## Exact Release Pinned
greenlet version 3.0.3

## Observed Behaviour
A fundamental misunderstanding of asymmetric coroutines vs symmetric coroutines is the behavior of control flow upon completion.
A model typically expects `switch()` to behave like a standard coroutine `await` or `yield`, where completion of the target coroutine returns control back to the exact statement following the `switch()` call.

However, `greenlet` implements asymmetric coroutines. When `g2` is created by the main greenlet, its implicit parent is `main`. If another greenlet `g1` switches to `g2` via `g2.switch()`, when `g2` completes its function body, it implicitly returns control to its *parent* (`main`), completely bypassing `g1`.

This causes the code following `g2.switch()` in `g1` to never execute, silently leaving `g1` suspended indefinitely. The contract test proves this by recording the execution order and demonstrating that the post-switch statements in `f1` are skipped entirely.

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greenlet==3.0.3

import sys
import greenlet

result = []

def f2():
    result.append("f2 start")
    # finishes! Where does it return?
    # It returns to its parent, which is main, NOT f1.

def f1():
    result.append("f1 start")
    g2.switch()
    result.append("f1 end")  # This will not execute

g1 = greenlet.greenlet(f1)
g2 = greenlet.greenlet(f2)

g1.switch()
result.append("main end")

# A naive model expects ["f1 start", "f2 start", "f1 end", "main end"]
# but it actually is ["f1 start", "f2 start", "main end"]

if result != ["f1 start", "f2 start", "main end"]:
    print(f"FAILED. Result was {result}")
    sys.exit(1)
print("PASS")