release: 1.2.0

[justinjoy]

Summary

• publish the first post-1.0 mainline release as 1.2.0
• synchronize Meson, generated version tests, README, changelog, pkg-config, and security support metadata
• preserve SONAME libchronoid.so.1 and document that 1.1.0 was development-only

Validation

• GCC 16.2.1 release build
• meson test -C build-release-1.2.0 --print-errorlogs (23/23 passed)
• installed version header, pkg-config metadata, license files, .so -> .so.1 -> .so.1.2.0, and SONAME verified
• footprint remeasured: shared 39,072 B stripped; static 55,532 B; CLI 37,248 B
• v1.0.2 installed public headers and exported dynamic symbols unchanged
• meson dist produced and inspected libchronoid-1.2.0.tar.xz