feat(cache): add cache_memory, an in-memory builder-driven cache

[ptomecek]

Summary

Adds cache_memory, a third caching primitive alongside cache and cache_parquet, closing the gap described in #26: an in-memory, builder-driven cache that does not require the plan to be serializable.

	input	key model	backend
cache	LazyFrame (plan)	serialize (content-addressed)	in-memory
cache_parquet	builder or plan (+ schema)	cache_path (explicit)	Parquet (disk)
cache_memory	builder or plan (+ schema)	none (instance / identity)	in-memory

cache derives its key by serializing the plan, so it cannot key a LazyFrame backed by register_io_source whose generator closes over non-picklable state (a connection, a lock, an open iterator). cache_parquet accepts a builder + schema and needs no serialization, but only has a disk backend. cache_memory fills the remaining corner: builder-callable input, no plan serialization, in-memory backend.

Behaviour

It collects the builder at most once into a closure-held buffer and replays it on every reference or collect, so a source read from many branches of a multi-branch evaluation (including pl.collect_all) executes upstream exactly once instead of once per reference. There is no key — reuse happens through the returned frame's identity, and the buffer is reclaimed by garbage collection when that frame is dropped.

• Construction and collect_schema() (with a callable schema) do no I/O and no collect; the builder runs only on first row demand.
• The one-time build is thread-safe; concurrent first-collects fanned out across pl.collect_all share a single build.
• The build outcome is terminal: any failure is recorded and re-raised on subsequent collects (as a neutral error carrying the original type and message), so a failing builder is not re-run per branch. Retry by constructing a fresh cache_memory.
• The collected frame is reconciled against the declared schema once (missing column or dtype mismatch raises; extras are dropped).

API

• Top-level: cache_memory(build_or_lf, *, schema)
• Namespace: lf.piot.cache_memory(*, schema)

Test plan

• New unit tests in tests/io_sources/test_lazy_cache_memory.py (23 cases): collect-once across references and pl.collect_all fan-out, thread-safe single build, buffer released on GC, non-serializable plugin source, callable-schema resolution without building, terminal/no-amplification failure semantics, no frame/traceback retention on failure, schema reconciliation, and empty-projection cardinality.
• ruff check and ruff format --check clean.
• Full io_sources suite passes.

Closes #26.

[github-actions]

Test Results

1 415 tests  +23   1 367 ✅ +23   39s ⏱️ -1s
    2 suites ± 0      48 💤 ± 0 
    2 files   ± 0       0 ❌ ± 0 

Results for commit c4462cd. ± Comparison against base commit 85adfbb.