docs(W4): revert channel-protocol to guide (F withdrawn), stamp docs/README.md as the contract's source of truth (Q1)

fred withdrew decision F after re-measuring channel-protocol.md with a control:
zero uppercase RFC2119 terms, seven lowercase 'must' all disclaiming authority,
under a banner refusing requirements status. F rested on 'the doc graph outranks
the page's own banner', which contradicts decision D, 'kind follows content'.
D is the rule; F was its counterexample. Neither rescue (vision's adjective edit
on the citing docs, or a banner edit) was taken: a kind that survives only by
editing the evidence around it is not a classification.

Q1 answered: docs/README.md prescribes the contract, so by D its content is a
spec, and by B it is the source of truth for that contract. A plan never
outranks the artifact it planned, so the flatten plan's stale 7-value enum is
corrected there rather than competing here.

128 live docs = 108 stamped + 16 operator-held + 3 supersede deferrals + 1 generated.

docs/README.md

@@ -1,3 +1,9 @@
+---
+kind: spec
+source_of_truth: true
+status: active
+---
+
 # Mosaic Stack Documentation
 
 This directory is the canonical home for Mosaic Stack product, architecture, API, operations, and delivery documentation.