Make strata consumable: install rules, exported target, usage docs

[lgoyal6]

Why

strata had no install rules, no namespaced target and no package config, so the only way to use it from another project was to vendor the source tree and reverse-engineer the include paths. It also had 0 tags and 0 releases, so there was no version anyone could pin.

What

• Export a strata::strata target that behaves identically through FetchContent and find_package, so a consumer can switch between them without touching their code.
• Public include dir now uses BUILD_INTERFACE / INSTALL_INTERFACE generator expressions. The old plain source path is meaningless once installed and would have made the exported target unusable.
• Install strata_warnings alongside. It is build-time-only, but the export set names it, so omitting it makes a consumer's find_package fail on a target it has never heard of.
• Tests, tools, fuzzers and the RocksDB benchmark now default OFF unless strata is the top-level project. A consumer has no use for the crash harness, and building it would break their configure on a missing RocksDB or a non-clang compiler.
• README gains a "Use it in your project" section.

A doc correction included here

The durability note I first wrote claimed WriteOptions{.sync = true}. That is wrong: WriteOptions is an empty struct. The guarantee is governed by Options::fsync_policy, which defaults to FsyncPolicy::kAlways. Fixed before commit.

Verification

Two throwaway consumer projects built outside the repo, one per path:

Path	Result
find_package(strata 0.1 REQUIRED) after cmake --install	builds, runs, read back: it links
FetchContent + strata::strata	builds, runs, read back: it links
Consumer build artifacts	libstrata.a only, no tests or tools, so the option defaults hold
strata's own build	tools still build, 46/46 tests pass
gtest leaking into a consumer prefix	no, INSTALL_GTEST OFF was already set here