feat(filter): receiver-side protect/risk engine for dest-only entries

- new bounded config-wire block (BLOCK_PROTECT_RULES) serializes the sender's
  compiled filter rules to the receiver (bounded count + 256 KiB patterns;
  strict action/sides validation)
- receiver evaluates protect/risk in the whole-tree extras walk and the
  per-directory delete plans, so a dest-only entry matching 'P' is kept like
  rsync; dry-run would-delete enumeration also honours it
- --filter flips to parity (115/11/31); per-dir merge receiver re-derivation
  remains the documented residual

src/shared/filter.h

@@ -53,7 +53,7 @@ typedef struct {
   char* pattern;       /* cleaned glob pattern (no leading '/', no trailing '/') */
 } FilterRule;
 
-typedef struct {
+typedef struct FilterRuleList {
   FilterRule** items; /* owned array of rule pointers */
   int count;
   int capacity;