docs(calibration.js): explain why one UTF-8 repair pass suffices in JS

TextEncoder always emits well-formed UTF-8, so truncation can strand at
most a lead byte plus three continuations — one repair pass covers it.
The C++ hub needs a loop because its input is raw bytes off the wire.
Also drops a dead variable from the byte-boundary test.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Client/udpstreamer/static/calibration.js

@@ -53,6 +53,13 @@
       //   Drop continuation bytes (10xxxxxx) from the tail until the last byte
       //   is either an ASCII byte (< 0x80) or a lead byte whose sequence is
       //   complete (i.e. all expected continuation bytes are present).
+      //
+      // A single pass suffices here, where the C++ needs a loop. The C++ input
+      // is a raw const char* straight off the wire and may hold arbitrary bytes;
+      // `bytes` here comes from TextEncoder, which always emits well-formed
+      // UTF-8 (unpaired surrogates become U+FFFD = EF BF BD, and no byte is ever
+      // >= 0xF8). Truncating well-formed UTF-8 can therefore strand at most a
+      // lead byte plus three continuation bytes, which one pass fully repairs.
       var b = bytes.slice(0, MAX_UNIT_LEN);
       var len = b.length;
       // Walk back over continuation bytes (up to 3) to find the lead byte of