core: in-memory ledger mode for testing

[Dexploarer]

What I want

const ledger = openLedger({ project: "test", inMemory: true });

Useful for:

• Unit tests that don't want to touch the filesystem.
• Sharing one in-memory ledger between an agent under test and Studio in the same process.
• CI environments where the default ~/.actantdb path doesn't exist or isn't writable.

Current state

openLedger requires a path. Tests work around this by writing to tempfile directories, which is fine until you want Studio to read the same in-memory store.

Implementation

node:sqlite supports :memory:. The Ledger constructor would need to:

• Accept inMemory: true and skip the mkdirSync + path-derivation.
• Pass :memory: to new DatabaseSync(":memory:").
• The same in-process Ledger instance is safe to share; cross-process in-memory sharing isn't.