feat(mcp): key-value scratchpad tools + database-targeted KV API

[StefanSteiner]

Summary

Adds a key-value scratchpad to the HyperDB MCP so an LLM can stash
variables, state, summaries, or JSON strings under a store/key
namespace without creating a table — and promotes the underlying
hyperdb-api KV API to a public, database-targetable surface to back it.

This is Milestone 2 of the KV-store work; Milestone 1 (the
hyperdb-api KvStore itself) shipped in #182. Both land together as one
feat: release.

What's in it

hyperdb-api (public API)

• KvStore::kv_store_in(database, name) and
  Connection::kv_list_stores_in(alias) — open / enumerate a KV store in a
  specific database. kv_store_in takes an unescaped database name
  and quotes it internally, so the API is safe by construction (the caller
  can't forget to escape). The async client gets the matching
  AsyncConnection::kv_store_in.

hyperdb-mcp (8 new tools)

Tool	Purpose
kv_set	Save a value under store + key (upsert)
kv_get	Read a value by store + key ({found, value})
kv_delete	Delete a key ({deleted})
kv_list	List keys in a store (sorted)
kv_list_stores	List store namespaces in a database
kv_size	Count keys in a store
kv_pop	Atomic read-and-remove of the lowest key (work-queue)
kv_clear	Delete all keys in a store

• Ephemeral by default. Omit database and the store lives in the
  ephemeral primary DB (lost on restart). Pass database:"persistent" (or
  persist:true) to persist across restarts, or any writable attached
  alias to target that database. kv_set's description carries a loud
  durability warning. Each database keeps its own isolated set of stores.
• Read-only aware. The four mutators (kv_set, kv_delete, kv_pop,
  kv_clear) are blocked with a read-only violation when the server runs
  --read-only; the four readers keep working.
• hyper://schema/kv resource documents the _hyperdb_kv_store
  backing table, its indexless shape, the ephemeral-vs-persistent
  durability rule, per-database isolation, and a LEFT JOIN template for
  enriching analytical tables with KV metadata.

Testing

• New: hyperdb-mcp/tests/kv_tools_tests.rs — 11 end-to-end tests
  driving the tools through the real rmcp dispatch path: CRUD lifecycle,
  key sorting, delete/pop/clear semantics, database routing + isolation
  (persistent vs ephemeral vs attached alias), the read-only guard
  (mutators blocked, readers allowed), the ephemeral-only guard
  (database:"persistent" errors, not panics), and persistence across a
  server restart.
• New: hyperdb-api/tests/kv_store_in_tests.rs — covers the
  database-targeted API.
• readme_tests.rs extended so all 8 tool names are enforced present in
  the LLM-facing README.
• Full workspace test suite green; cargo clippy --workspace --all-targets --all-features -- -D warnings clean; make doc introduces no new
  warnings; real-tooling stdio smoke test confirms the 8 tools appear in
  tools/list, hyper://schema/kv is listed and readable, and a
  kv_set→kv_get→kv_list round-trip works against the release binary.

Changelog

• hyperdb-api/CHANGELOG.md — the kv_store_in / kv_list_stores_in
  public API under ### Added.
• hyperdb-mcp/CHANGELOG.md — the 8 kv_* tools and the
  hyper://schema/kv resource under ### Added.

[StefanSteiner]

Follow-up: filed #192 with 5 improvements to these KV tools, surfaced by dogfooding them in a real Claude Code session (storing + querying a 70 KB ~/.claude.json blob) — value_path for file-backed kv_set, an insert-vs-overwrite signal, value-size reporting, batch ops (the API-layer set_batch already exists, just unsurfaced), and docs for the value::json + JSON_EACH query pattern. Each item has a proposed fix and a code citation.