Final not honored when applied post-hoc

Bug Report

The behavior of Final is a bit inconsistent. It can be applied post-hoc, but then it won't be honored.

To Reproduce

import platform
import sys

from typing import Final

SERIES_STD = ' ▌█' if platform.system() == 'Windows' else ' ▏▎▍▌▋▊▉█'
if sys.__stdout__ and sys.__stdout__.encoding:
    try:
        SERIES_STD.encode(sys.__stdout__.encoding)
    except UnicodeEncodeError:
        SERIES_STD = ' |'

SERIES_STD: Final = SERIES_STD
SERIES_STD = '| '

Expected Behavior

I expect an assignment error for the final line to raise an error.

error: Cannot assign to final name "SERIES_STD"  [misc]

Some people might argue applying Final post-hoc itself should raise an error, but I'm not one of them.

Actual Behavior

Success: no issues found in 1 source file

Your Environment

• Mypy version used: 1.18.2
• Mypy command-line flags: None
• Mypy configuration options from mypy.ini (and other config files): No Config
• Python version used: 3.13.9

Start by running the supplied reproducer with mypy 1.18.2 and trace how post-hoc Final annotations are handled. Identify the relevant type-checking path, add coverage for this example, and confirm that the reassignment reports "Cannot assign to final name" while other Final behavior remains unchanged.