v1.2.1 — Translate comments and debug strings to English

i18n cleanup — no API change.

• 26 source files + 8 example sketches
• ~220 comment lines translated FR → EN (banners, doc-comments, inline)
• ~12 Serial.print(F(...)) debug strings translated (gated behind INSTANT_DEBUG)
• Zero code identifier renamed, zero logic change

If you compile any v1.2.0 sketch with v1.2.1 → identical binary output.