docs: mark the legacy pipeline as frozen

This module is the pre-2.0 implementation, kept so older workflows reproduce
their seeds. Its behaviour is pinned byte for byte by the golden tests, so the
header says plainly that it takes crash fixes only and that new work belongs in
core/ and pipelines/standard.py -- including why the inert visualiser and
debugger calls throughout are deliberately left alone.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

shader_noise_ksampler.py

@@ -1,3 +1,22 @@
+"""
+The legacy sampling pipeline. FROZEN -- crash fixes only.
+
+This is the pre-2.0 implementation, kept so that workflows saved before 2.0
+reproduce their seeds exactly. `sampling_mode: legacy` runs this code; the
+default, `standard`, runs pipelines/standard.py instead.
+
+Do not refactor or tidy this module. Its behaviour is pinned byte for byte by
+tests/test_legacy_golden.py (11 recorded configurations) and was verified
+pixel-identical across 12 rendered configurations on SD1.5 and Wan 2.1. That
+includes the parts that look like dead weight: the visualiser and debugger
+calls throughout are stubs from core/debug.py that can never be enabled,
+and rewriting the ~163 call sites would risk the guarantee for no gain.
+
+New work belongs in core/ and pipelines/standard.py. What this pipeline gets
+wrong -- stages restarting from maximum noise, `denoise` and `custom_sigmas`
+being ignored, blend modes shifting the noise distribution -- is documented in
+CODE_REVIEW.md and fixed there.
+"""
 import torch
 import torch.nn.functional as F
 import math