Add built-in octave shift feature

[fsecgin]

Summary

• Adds an octaveShift property to rtttl_context_t that shifts all note frequencies up or down by N octaves before calling tone()
• Default is 0 (no shift), fully backward compatible, no breaking changes
• Eliminates the need for users to write a custom setToneFunction wrapper just to shift pitch for passive buzzers

Motivation

Passive buzzers have a resonant frequency sweet spot (often 2500Hz+), but RTTTL melodies are written for the standard musical range (200-800Hz). Currently the only way to shift frequencies is to override setToneFunction with a wrapper that multiplies the frequency before passing it to tone(). This is boilerplate that every passive buzzer user ends up writing.

API

// Global context (legacy API)
anyrtttl::setOctaveShift(2);  // shift up 2 octaves

// Specific context (multi-speaker)
anyrtttl::setOctaveShift(myContext, 2);

Changes

• src/anyrtttl.h: Added int8_t octaveShift field to rtttl_context_t, added setOctaveShift() declarations
• src/anyrtttl.cpp: Implemented setter, initialized field in initContext(), applied shift at frequency lookup in nextNote(), preserved shift across begin() calls
• examples/OctaveShift/OctaveShift.ino: Example showing normal vs shifted playback
• README.md: Documentation section with usage example
• CHANGES: Entry for v2.7.0

Test plan

• Clean build on ATmega32U4 (SparkFun Pro Micro 16MHz) via PlatformIO
• Verified same RAM/Flash footprint as unmodified library (338B / 7810B)
• Tested with passive buzzer, confirmed shifted playback matches previous setToneFunction wrapper behavior
• Should also compile on ESP32 and ESP8266 (not tested yet)