source: reduce O3 binary footprint

[justinjoy]

Summary

• restore Meson standard release O3 and remove the unreleased compact_release override from build: reduce release artifact size #17
• compact the private RNG-state observation helper by using its fixed-capacity test contract
• report standard-O3 footprint and section sizes in main CI

Size (GCC 16.2.1, x86_64, standard O3)

Artifact	Before	After	Reduction
stripped shared	43,168 B	39,072 B	9.5%
static archive	56,142 B	55,532 B	1.1%
chronoid-gen	37,312 B	37,248 B	0.2%

The historical #17 O2 configuration measured 39,072 / 52,358 / 33,328 B. As a diagnostic, the same source change at explicit O2 measures 34,976 / 51,748 / 33,256 B; the shipping configuration in this PR remains standard O3.

Validation

• GCC and Clang: 23/23 tests
• GCC and Clang: AVX2-disabled, SSSE3-disabled, and SIMD-none lanes
• runtime forced-scalar batch tests
• gst-indent and clang-tidy
• exported symbols, SONAME, and runtime dependencies unchanged
• 9-sample interleaved medians: random +1.6%, KSUID -1.6%, UUIDv7 -0.3%

Follow-up to #17.