Document frontend branch protection activation (#175)

## Purpose

After-activation canary for repository ruleset `19051697`.

## Evidence before activation

- preparation PR #172 established stable `Required CI`
- strict-current-base remediation PR #174 passed `Required CI` and
Qodana
- merged master run `29501928124` completed with 43 successful jobs and
one intentional skip
- effective master rules were audited after activation

## Canary acceptance

- merge is blocked while `Required CI` is pending
- the exact `Required CI` check comes from GitHub Actions integration
`15368`
- only squash merge is accepted
- the branch is deleted automatically after merge

.github/BRANCH_PROTECTION.md

@@ -35,6 +35,14 @@ pull-request reviews.
 If validation exposes a blocker, disable the ruleset rather than deleting it so
 its configuration and history remain available.
 
+## Activation record
+
+Repository ruleset `19051697` was activated on 2026-07-16 after preparation
+PR #172 established `Required CI`, remediation PR #174 passed the strict
+current-base gate, and merged master run `29501928124` completed with all 43
+executed jobs successful. This documentation update is the after-activation
+canary for the normal protected pull-request path.
+
 ## Normal publishing flow
 
 Create a scoped feature branch, open a pull request to `master`, wait for