Deprecate size parameter on WorkerPool in favor of spawn

[conradbzura]

Summary

Rename the size parameter on WorkerPool to spawn across all three __init__ overloads and internal implementation. The size/lease parameter pair read ambiguously — size could refer to total pool capacity rather than the number of spawned workers. The new spawn name makes the semantics explicit: spawn is how many workers the pool starts, lease is how many additional discovered workers it accepts.

WorkerPool(size=N) continues to work at runtime via a DeprecationWarning, and type checkers (pyright, mypy) flag it at static analysis time via PEP 702 @deprecated overloads. Passing both size and spawn raises TypeError. All internal references (_resolve_spawn, _worker_context, match arms) and README documentation use spawn as the canonical name.

Closes #134

Proposed changes

Rename size to spawn in the implementation signature

Replace size with spawn in all three @overload signatures (ephemeral, durable, hybrid) and the implementation body. Rename the _resolve_size helper to _resolve_spawn and update its error message from "Size must be non-negative" to "Spawn must be non-negative". The _worker_context method parameter and all match-arm bindings now use spawn.

Add runtime deprecation bridge for size

The implementation signature accepts both spawn and size as optional parameters. When size is provided alone, emit a DeprecationWarning at stacklevel=2 (pointing to the caller's frame) and forward the value to spawn. When both are provided, raise TypeError.

Update README documentation

Replace size with spawn in all code examples across the worker, discovery, and load-balancer READMEs. Update the mode table to use spawn as the column header and rewrite the spawn/lease explanation for clarity.

Test cases

#	Test Suite	Given	When	Then	Coverage Target
1	TestWorkerPool	The deprecated size parameter is passed	WorkerPool is initialized	Emit a DeprecationWarning	Runtime deprecation warning
2	TestWorkerPool	The spawn parameter is passed	WorkerPool is initialized	No DeprecationWarning is emitted	Clean path for new API
3	TestWorkerPool	Both size and spawn parameters are provided	WorkerPool is initialized	Raise TypeError	Mutual exclusion guard
4	TestWorkerPool	The deprecated size parameter with value 4	WorkerPool is initialized	Create a valid pool identical to spawn=4	Value forwarding
5	TestWorkerPool	The deprecated size parameter with value -1	WorkerPool is initialized	Emit a DeprecationWarning and raise ValueError	Negative value forwarding
6	TestWorkerPool	The deprecated size parameter with value 0 and CPU count available	WorkerPool is initialized	Emit a DeprecationWarning and create a valid pool	Zero/CPU-count resolution
7	TestWorkerPool	The deprecated size parameter and a discovery service	WorkerPool is initialized	Emit a DeprecationWarning and create a valid pool	Hybrid mode compatibility
8	TestWorkerPool	The deprecated size parameter with value 4 and lease of 8	WorkerPool is initialized	Emit a DeprecationWarning and create a valid pool	Lease combination
9	TestWorkerPool	The deprecated size parameter is passed	WorkerPool is initialized	Warning filename points to the test file	Stacklevel correctness