Release v2.5.0 - #119

Version and changelog only. No library code, license, or behavior change. src/si5351.cpp and src/si5351.h are untouched, so this is safe to merge and then tag.

library.properties and library.json are both 2.5.0. The README changelog follows the v2.4.2 release: a new * v2.5.0 section above * v2.4.2.

Reviewed since tag v2.4.2 (f885fca):

• Add v2.4.2 guard tests ahead of flash refactor #116 is host regression and the Uno flash budget only. It does not change the library, so it is not a changelog bullet.
• Reduce flash size: optional CLK6/7 gate and shared PLL/init code #117 shares one PLL program path and the init()/reset() loops (same register programming as v2.4.2, smaller default flash) and adds the SI5351_NO_CLK67 opt-out.
• Add SI5351_NO_VCXO opt-out for flash savings #118 adds the SI5351_NO_VCXO opt-out and changes set_vcxo() from void to uint8_t.

Do not tag 3ca004c (merged #118). That commit still says 2.4.2. After this PR is merged, tag v2.5.0 on the merge commit into master, the same way v2.4.2 is the merge of #115. Suggested release title: v2.5.0. Suggested notes:

• Smaller default AVR flash, with the same register programming as v2.4.2: one shared PLL program path, and one copy of the init()/reset() output loops. On an Uno (arduino-cli 1.5.1, arduino:avr 1.8.6) the minimal init() + set_freq() sketch is 10,140 bytes for a constant CLK0 (was 10,830) and 10,868 bytes for a runtime clock index (was 11,558)
• Leave out CLK6/CLK7 with #define SI5351_NO_CLK67 before #include <si5351.h>, or -DSI5351_NO_CLK67. set_freq() and set_freq_manual() return 1 for CLK6 and CLK7 and write nothing, and set_ms() writes nothing for those outputs. The si5351_clock enumerators stay, and reset() still initializes all eight clock-control registers. About 1 KB less flash on that Uno runtime-clock sketch (9,870 bytes with the sketch define, 9,836 with the compiler flag)
• Leave out the Si5351B VCXO the same way with SI5351_NO_VCXO, alone or with SI5351_NO_CLK67. set_vcxo() returns 1 and writes nothing, and later PLL updates use the non-VCXO denominator. reset() still clears the VCXO parameter registers. On that same sketch the define saves 440 bytes (10,428) and the compiler flag saves 472 bytes (10,396). Both defines together are 9,390 bytes
• set_vcxo() returns uint8_t: 0 when the PLL and VCXO registers are written, 1 when nothing is written (reference not usable, or SI5351_NO_VCXO). Sketches that ignore the return still compile. Without the macro, register writes match v2.4.2

Local check before push: run_v231_checks.sh --src --require-fixed on this tree, OVERALL PASS, including 0 differing runs vs pinned v2.4.2. The pinned source hash of si5351.cpp/si5351.h is unchanged (0934075c6b6b5daf). library.properties reported version 2.5.0.