refactor: extract KVP into standalone libazureinit-kvp crate with layered architecture

[peytonr18]

Summary

Extracts KVP into a standalone crate, libazureinit-kvp, and replaces the legacy libazureinit::kvp module with a layered, trait-generic architecture.

This keeps azure-init's tracing-based KVP emission behavior while making KVP reusable for external distros without pulling in the full azure-init stack.

• Replaces libazureinit/src/kvp.rs (902 lines) with a new workspace crate: libazureinit-kvp.
• Moves KVP from async channel/background writer semantics to synchronous flock-guarded I/O.
• Preserves existing tracing integration in azure-init while simplifying runtime lifecycle management.
• Adds architecture and API documentation for internal and external consumers.

What Changed

New crate: libazureinit-kvp

Introduces a 4-layer design plus top-level client wiring:

• Layer 0: KvpStore trait
  • HyperVKvpStore (production pool-file implementation)
  • InMemoryKvpStore (test double)
• Layer 1: DiagnosticsKvp<S>
  • typed diagnostics
  • Azure 1022-byte value chunking support
• Layer 2: TracingKvpLayer<S>
  • tracing_subscriber::Layer adapter
  • handles tracing events/spans and health_report passthrough
• Layer 3: ProvisioningReport
  • typed accessor for PROVISIONING_REPORT
• Top-level: Kvp<S> client to wire all layers together

Legacy module removed

• Deletes libazureinit/src/kvp.rs; functionality now lives in libazureinit-kvp.

libazureinit integration simplified

• libazureinit/src/logging.rs
  • removes old KVP wrapper/lifecycle plumbing (JoinHandle, CancellationToken, halt(), close())
  • setup_layers now returns only Box<dyn Subscriber + Send + Sync>
• src/main.rs
  • removes KVP shutdown orchestration block
  • no explicit async KVP lifecycle handling needed

Dependency updates

• libazureinit drops direct fs2/sysinfo usage for KVP internals
• adds path dependency on libazureinit-kvp
• workspace now includes libazureinit-kvp

Docs

• Adds doc/kvp_design.md (design rationale + architecture)
• Rewrites doc/libazurekvp.md (API/reference)
• Aligns docs with implemented key format and ProvisioningReport API
• Documents/reporting behavior for long health_report values

Why

• External reuse: other distros can consume KVP directly with minimal dependencies.
• Separation of concerns: storage, diagnostics, tracing adapter, and provisioning report logic are decoupled.
• Simpler lifecycle: no channel draining, shutdown tokens, or background writer task ownership.
• Testability: each layer can be tested independently; in-memory testing is straightforward.

Behavioral Notes

• KVP writes are now synchronous (flock + write + unlock) rather than buffered async channel writes.
• Long diagnostic/report values are chunked to account for Azure's read limits.
• health_report to PROVISIONING_REPORT path preserves long values through chunked writes.

Architecture

┌─────────────────────────────────────────────────┐
│ Kvp<S> (top-level client, wires layers)         │
├─────────────────────────────────────────────────┤
│ Layer 3: ProvisioningReport                     │
│          Typed accessor for PROVISIONING_REPORT │
├─────────────────────────────────────────────────┤
│ Layer 2: TracingKvpLayer<S>                     │
│          tracing_subscriber::Layer adapter      │
├─────────────────────────────────────────────────┤
│ Layer 1: DiagnosticsKvp<S>                      │
│          Typed diagnostic events + chunking     │
├─────────────────────────────────────────────────┤
│ Layer 0: KvpStore trait                         │
│   ┌──────────────────┐   ┌─────────────────────┐│
│   │ HyperVKvpStore   │   │ InMemoryKvpStore    ││
│   │ (production)     │   │ (test double)       ││
│   └──────────────────┘   └─────────────────────┘│
└─────────────────────────────────────────────────┘

See doc/kvp_design.md for design rationale and doc/libazurekvp.md for API details.

Fixes #239. Supersedes #286.