build: prepare v1.0.0

[edsiper]

Summary

Prepare CFL 1.0.0 and add release validation for installed and downstream consumers.

• set the project and package version to 1.0.0
• add the initial changelog and link it from the README
• install the bundled xxHash archive alongside its public headers
• compile and run a standalone consumer against the installed artifacts
• add cprofiles to the downstream validation matrix
• document mandatory downstream impact checks for future CFL changes

Rationale

CFL is consumed both as a bundled CMake dependency and as an installed static library. The previous install set contained CFL and xxHash headers but omitted the xxHash archive required by CFL's public hash interface. The installed-consumer check exercises initialization, containers, version reporting, and hashing to prevent packaging regressions.

The release validation also covers all primary telemetry consumers: Fluent Bit, cmetrics, ctraces, and cprofiles.

Local validation

• CFL GNU99: 34/34 tests passed
• CFL GNU17: 34/34 tests passed
• installed external consumer: built, linked, and ran successfully
• cmetrics with CFL 1.0.0: 20/20 tests passed
• ctraces with CFL 1.0.0: 4/4 tests passed
• cprofiles with CFL 1.0.0: 4/4 tests passed
• Fluent Bit with CFL 1.0.0: full build passed, including content modifier, metrics selector, SQL, and sampling processors
• git diff --check passed

The v1.0.0 tag will be created only after this PR is merged and the final master validation is green.