Instrument KV operation timing logs

[ChristianPavilonis]

Summary

• Add debug-level timing logs for KvHandle backend operations across adapters.
• Log only safe KV metadata so operation latency is observable without exposing raw keys, prefixes, cursors, or values.
• Document KV operation timing behavior in the guide.

Related external tracking: IABTechLab/trusted-server#731

Changes

Crate / File	Change
crates/edgezero-core/src/key_value_store.rs	Times KvHandle backend calls with web_time::Instant and emits guarded log::debug! metadata for get/put/TTL/raw/list/delete/exists operations.
docs/guide/kv.md	Adds an Operation Timing / Observability section describing debug logs, safe metadata, typed helper timing boundaries, and read_modify_write behavior.

Closes

Closes #271

Test plan

• cargo test --workspace --all-targets
• cargo clippy --workspace --all-targets --all-features -- -D warnings
• cargo check --workspace --all-targets --features "fastly cloudflare spin"
• WASM builds: wasm32-wasip1 Spin via cargo check -p edgezero-adapter-spin --target wasm32-wasip1 --features spin
• Manual testing via edgezero-cli dev
• Other: cargo test -p edgezero-core; cargo test -p edgezero-adapter-axum --all-targets; cargo fmt --all -- --check

Checklist

• Changes follow CLAUDE.md conventions
• No Tokio deps added to core or adapter crates
• Route params use {id} syntax (not :id)
• Types imported from edgezero_core (not http crate)
• New code has tests
• No secrets or credentials committed