fix: filter out partial name substitutions from reveal pairs

The smart engine records individual name parts ("Ademo"→"John",
"Demo"→"Smith") AND the combined form ("Ademo Demo"→"John Smith")
in sessionSubstitutions. The catch-all in buildRevealPairs was
adding all of them, causing partial replacements that corrupted
the DOM and made the cache oscillate between 4 and 0 pairs.

Fix: skip session entries whose key is a substring of a longer
entry (e.g. "ademo" is part of "ademo demo"). Only the combined
form gets added as a reveal pair.

Also: remove debug logging, improve cache with size tracking.

https://claude.ai/code/session_01KF4i7Ra7zCEDskxDBaNtcT

manifest.firefox.json

@@ -1,7 +1,7 @@
 {
   "manifest_version": 3,
   "name": "Silent Send",
-  "version": "0.9.15",
+  "version": "0.9.16",
   "description": "Intercepts personal info and substitutes it with user-defined replacements before sending to AI services.",
   "browser_specific_settings": {
     "gecko": {