Add C# lock-free shared-memory store profile (v2.0.0)

[rantri]

Summary

• add an explicitly selected C# lock-free shared-memory key-value profile using mapped 64-bit atomics
• add layout 2.0/resource protocol 2 with generation-fenced directory, slot, lease, participant, recovery, removal, and reuse state
• support zero-copy publication through reserve/write/advance/commit and shared zero-copy read leases
• preserve the existing MemoryStore facade, Legacy default profile, layout 1.2 behavior, status numbers, and bounded cold-lifecycle OS coordination
• add the broker-key sample, protocol documentation, diagnostics, linearizability coverage, crash/recovery testing, benchmarks, and qualification tooling
• advance the managed NuGet package to 2.0.0; native CMake and Python remain independently versioned 0.1.0 and deliberately reject layout 2.0

Why

The Legacy profile acquires a named cross-process semaphore on each store operation. Under multi-process load this serializes otherwise independent operations and makes one delayed process affect all peers. The new profile removes OS synchronization from steady-state publish, acquire, release, remove, and maintenance paths while keeping SharedMemoryStore a bounded general-purpose key-value store. Broker scheduling and load balancing remain outside the library.

Compatibility and rollout

• Lock-free mode is opt-in through SharedMemoryStoreOptions.CreateLockFree(...).
• Legacy remains the default.
• Layout 1.2 and 2.0 mappings are distinct; upgrade or rollback requires drain, close, recreate, and application-owned republish.
• C++ and Python remain layout-1.2-only in this release.

Validation

• post-merge Release suite: 1,028/1,028 passed, zero skipped
  • Contract 117
  • Linearizability 83
  • Interop 75
  • Unit 451
  • Integration 302
• PR qualification: 24/24 gates passed
• nightly qualification: 24/24 gates passed, including 8,000,000 production races, 10,000,000 churn cycles, and 10,000 crash/recovery cases
• Linux x64 validation: all 28 required gates passed
• independent code/spec/evidence reviews: H0/M0/L0
• documentation validation passed
• packed SharedMemoryStore.2.0.0.nupkg and symbol package successfully

Linux x64 eight-process results on the frozen source tree:

• acquire/release: 22.33M lock-free calls/s versus 0.252M Legacy (88.6x)
• publish/remove: 6.67M versus 0.954M (6.99x)
• lock-free p99: 1.4 us acquire/release and 4.5 us publish/remove

Qualification caveat

The deliberately exhaustive terminal release tier was stopped because its repeated multi-hour matrix was not suitable as a blocking developer gate. It did not produce a final release summary, so this PR does not claim formal exhaustive-release qualification. The completed PR, nightly, Linux, full-suite, package, and review evidence supports implementation review and a controlled C# x64 rollout; repository CI should still run on this PR.