Memoize proxy locations in produce() with epoch invalidation

[Korijn]

Track E, PR 10 (foundational) — the produce() hot-path design pass.

What

produce()'s proxies compute their path by walking parent links to the root on every recorded write, so writes into deep trees paid an O(depth) walk plus a list build/reverse/tuple conversion each time.

_location() now memoizes its result per proxy and reuses the parent's memoized location recursively (so ancestors' caches are shared across siblings). Repeated writes into a stable tree pay O(1) per write.

Why it stays correct

The whole point of parent-link paths is that proxies move (list shifts, adoption). Correctness is preserved by an epoch counter on the PatchRecorder: every structural change that can move or orphan a proxy bumps it, invalidating all memoized locations at once —

• _detach / _detach_all (removals, replacements)
• re-attach through _adopt (move semantics)
• _shift_cache (list index shifts from insert/del/slice ops)
• sort() / reverse() re-keying
• finalize's root detach (so a leaked proxy can't record through a stale cache)

Proxy chains are strongly held through the _proxies registries while the draft is alive, so a parent weakref can only die after finalize — which always bumps the epoch first. The produce test suite (live-paths, detach/adopt, proxy-leak modules) exercises all of these invalidation paths; 405 tests pass at 100% coverage, ruff and ty clean.

Benchmarks

The suite had no deep-tree produce cases, so this PR adds two (produce-deep group): repeated leaf writes at depth 12, and field updates across a list of nested dicts. Interleaved medians vs master, 2 runs each, PYTHONHASHSEED=0, GC off:

benchmark	master	this PR	change
produce_deep_leaf_writes[copy]	173.9µs	109.1µs	−36.3%
produce_deep_leaf_writes[in_place]	167.3µs	103.6µs	−37.4%
produce_deep_nested_mutation (1 write/call)	35.6µs	36.8µs	+2.4% (pays the cache store without reusing it)
flat dict / list / set / observ workloads	—	—	within noise (±2%, worst single case +3.8%)

Docs

The Internals produce section now describes the memoization and its invalidation rule.

🤖 Generated with Claude Code

https://claude.ai/code/session_016Mu9vEBwU4fQLi8ZgkgdS2

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Generated by Claude Code

[Korijn]

@berendkleinhaneveld I'll let you review & approve this one