docs: changelog entry for #70's native build flags

[dfa1]

Summary

Adds an [Unreleased] section to CHANGELOG.md covering the changes that landed from #70's investigation:

• Added: legacy format decode v0.4-v0.7 (build: support decoding legacy zstd formats v0.4-v0.7 #73)
• Changed: BMI2 asm decode path, ARM CRC baseline (build: native build flags from #70 (asm, arch baseline, hardening) #71)
• Security: RELRO + immediate binding on Linux (build: native build flags from #70 (asm, arch baseline, hardening) #71)
• Fixed: Windows native build (fix: build native libs via bash, not direct .sh execution #75)

Also notes what was investigated and rejected (LTO, x86-64-v3) with a pointer to the full benchmark data on #70, so the "why not" is discoverable from the changelog without digging through issue history.

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