RFC/draft: opt-in libSQL/Turso storage backend for concurrent multi-writer memory

[Donach]

Stacked on #320 (Nix flake packaging). This branch includes #320's commit, so the diff here shows flake.nix/flake.lock/README too until #320 merges — after that this PR shrinks to just the backend (+ a small icm-turso flake package output added here). Review #320 first if you're taking both; this PR works without the flake if you'd rather drop #320.

Summary — opt-in libSQL/Turso storage backend (4th additive backend)

ICM keeps memory in a single local SQLite file, so it can't be shared or written
concurrently from more than one machine/process. This adds an opt-in Turso/libSQL
backend that runs the existing icm-store SQL over the async libsql client, so
the store can live in a libSQL/Turso database — local file, a remote sqld/Turso
server, or an embedded replica. A remote server is the multi-writer path (serialises
writes from every ICM process).

The default build is completely unchanged — rusqlite, exactly as today.

Update: now using shared-source (#[path]) — upstream-mergeable

The original draft duplicated ~7,000 lines (turso_store.rs + turso_schema.rs).
This revision eliminates the copies entirely via Rust's #[path] attribute:

// sqlite_backend.rs — compiles store.rs/schema.rs with real rusqlite
pub(crate) mod sql { pub use rusqlite::{params, Connection, ...}; }
#[path = "schema.rs"] pub(crate) mod schema;
#[path = "store.rs"]  pub(crate) mod store;

// turso_backend.rs — same files with the libsql sync facade
pub(crate) mod sql { pub use crate::dbcompat::{params, Connection, ...}; }
#[path = "schema.rs"] pub(crate) mod schema;
#[path = "store.rs"]  pub(crate) mod store;

Net change vs the original draft: −8,022 lines (302 inserted, 8,324 deleted).
store.rs and schema.rs are only touched by two mechanical changes:

• use rusqlite:: → use super::sql:: (provider alias)
• collect_rows<T> signature: MappedRows<'_, F> → impl Iterator<Item = sql::Result<T>>

This makes the backend a genuine minimal diff against upstream and easy to rebase going forward.

What's new since the original draft

• Fully rebased on v0.10.57 (76 upstream commits, including the additive-backend sprint feat(store): support pluggable storage backends (beyond SQLite) #301)
• Turso is the 4th additive backend alongside sqlite/postgres/opensearch — same dispatch! macro
• Auto-detect: if ICM_DB_BACKEND is unset but TURSO_DATABASE_URL or LIBSQL_URL is non-empty, turso is selected automatically (gated behind #[cfg(feature = "turso")])
• dbcompat.rs: 540-line synchronous rusqlite-shaped facade over the async libsql client
• Full-coverage: unlike postgres/opensearch, the Turso backend supports all of memoirs, concepts, transcripts, feedback, and facts (same SQL dialect as sqlite)

Feature flag / env vars

# Cargo.toml
[features]
turso = ["dep:libsql", "dep:libsql-ffi", "dep:once_cell", "dep:tokio", "dep:sqlite-vec", "dep:zerocopy"]

# Auto-detect (no ICM_DB_BACKEND needed):
export TURSO_DATABASE_URL=http://localhost:8080       # local sqld
export TURSO_DATABASE_URL=libsql://xyz.turso.io      # Turso cloud

# Explicit:
export ICM_DB_BACKEND=turso

Test results

cargo test -p icm-store --lib
  test result: ok. 190 passed; 0 failed; 2 ignored  (sqlite)

cargo test -p icm-store --no-default-features --features turso --lib
  test result: ok. 194 passed; 0 failed; 2 ignored  (turso)

All upstream tests pass on both backends. Smoke-tested against sqld 0.24.33:
memoir, memory, facts, feedback, transcript — all ✅.

Head branch

This PR's head has been updated to Donach:rebase/turso-on-upstream-v0.10.57
(the turso-backend-minimal branch on the Donach/icm fork is our production
branch and cannot be force-pushed — it is tracked by NixOS infrastructure).

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RFC — happy to reshape per your feedback before marking ready.