[POC] In-memory lmdb adapter for Pyodide/WASM

[jaelliot]

Proof of Concept / Demo

This is a volatile, in-memory lmdb API adapter that allows keripy's LMDBer to boot inside Pyodide (WASM) where the real lmdb C extension cannot load.

Status: Proof of concept — not production-ready. Data is lost on page refresh.

What it does

• Provides a dict-backed implementation of the lmdb API subset that keripy actually calls
• Enables LMDBer(name=..., temp=True, reopen=True) to succeed in Pyodide
• Covers: Environment, _Transaction, _Cursor, _Database, BadValsizeError
• Supports dupsort databases with sorted duplicate values

What it does NOT do

• No persistence — all data is in-memory Python dicts, lost on page refresh
• No IndexedDB integration — that's Phase 2 (wrapping this adapter with async load/save)
• No MVCC — single-threaded Pyodide doesn't need it

Validated against

• 44 unit tests covering all keripy dbing.py cursor patterns
• LMDBer integration test: instantiation, setVer/getVer, putVal/getVal/delVal, setVal, getTopItemIter, cntTop, remTop, cntAll, dupsort putVals/getVals/cntVals/getValLast/delVals, close

Bug found and fixed during integration

txn.delete(key, b'') on dupsort databases: real lmdb treats empty val as "delete all dups at key." The adapter initially treated b'' as a specific value lookup. Fixed to match real lmdb semantics.

Files changed

• python/lmdb.py — single new file, ~499 lines, zero external dependencies (only bisect from stdlib)