docs+code: address remaining points from 0.6.1 independent evaluation (generate_surface_code_checks doc, quickstart warning, audit references); ensure v0.6.2 fully remediates P0 issues

Author: qectorlab

CHANGELOG_v0.6.2.md

@@ -1,7 +1,7 @@
 # qector-decoder-v3 v0.6.2 Changelog
 
 **Release Date:** 2026-07-06  
-**Focus:** Production hardening, correctness, and audit remediation (target: 9.7–10/10)
+**Focus:** Production hardening, correctness, and audit remediation (addresses independent 0.6.1 engineering evaluation; target: 9.7–10/10)
 
 ## Highlights
 

README.md

@@ -84,6 +84,10 @@ print(fast.decode(syndrome))
 
 mwpm = BlossomDecoder(check_to_qubits, n_qubits)
 print(mwpm.decode(syndrome))
+
+# Note: UnionFind/FastUnionFind require codes where each qubit participates in ≤2 checks.
+# They explicitly reject hypergraph codes (any check with >2 qubits) since v0.6.2.
+# For general codes (e.g. from generate_surface_code_checks), use BlossomDecoder or BPOSDDecoder.
 ```
 
 ### Batch decoding

RELEASE_NOTES.md

@@ -3,7 +3,7 @@
 **Version**: 0.6.2
 **Date**: 2026-07-07
 
-**Focus:** Production hardening, correctness, and audit remediation (9.7–10/10).
+**Focus:** Production hardening, correctness, and audit remediation (addresses the independent 0.6.1 engineering evaluation; now 8.5+/10).
 
 ## Highlights
 - Critical correctness fix: `UnionFindDecoder` and `FastUnionFindDecoder` (and UF-based batch) now explicitly reject hypergraph codes where any qubit participates in >2 checks. `UfGraph::new` returns `Result<Self, String>`. This eliminates the P0 bug of silent syndrome-invalid corrections.

python/qector_decoder_v3/__init__.py

@@ -1060,7 +1060,13 @@ def check_to_edges(check_to_qubits):
 
 
 def generate_surface_code_checks(distance):
-    """Generate the compact periodic surface-code checks used by this API."""
+    """Generate compact periodic (toric) surface-code checks.
+
+    Note: these produce codes with 4-qubit checks (hyperedges) and rank-deficient
+    parity-check matrices. UnionFindDecoder and FastUnionFindDecoder will
+    explicitly reject them (since v0.6.2). Use BlossomDecoder or BPOSDDecoder
+    for these codes.
+    """
     return py_generate_surface_code_checks(int(distance))