Require a value-producing head before matching return tails

[EiNSTeiN-]

I am opening this PR to prevent a padded handler-return trailer from matching and swallowing the function's later return-value construction. The following source loses its final return build(item) when the handler's return bytecode contains inert padding:

def select(read, build):
    try:
        item = read()
    except ValueError:
        return build("missing")
    return build(item)

With NOPs in the return trailers, pycdc currently emits an incomplete function that ends at the exception clause:

def select(read, build):
    try:
        item = read()
    except ValueError:
        return build("missing")

Some returns carry a value computed before cleanup. Treating only padding plus RETURN as a duplicated value-building tail can match unrelated mainline returns, swallowing their call or response construction and leaving an empty stack.

The change makes sunk_return_mirror_merge skip known padding to find the first meaningful operation and require a genuine value head: a load or an empty container construction. A carried-value return is left for the existing scope/value reconstruction. An actual independently constructed return tail remains eligible for matching.

The same issue applies to the asynchronous variation, where item = await read(). This matcher operates on Python 3.11+ exception-table layouts. The padding preserves jump targets and exception-table boundaries; it does not change which source path should return a value.