Add opt-in Wired Memory

[ronaldmannak]

Add an opt-in Wired Memory execution boundary for continuous batching.

BatchGenerator now has a drain(wiredMemoryTicket:onResponse:) convenience method that holds a WiredMemoryTicket across the full drain loop, rather than starting and ending the ticket for each next() step. The low-level next() API remains synchronous.

Changes

• Added BatchGenerator.drain(wiredMemoryTicket:onResponse:).
• Kept the ticket active until all queued/active rows are drained, the response handler throws, or the task is cancelled.
• Added cancellation checks before each generator step and before each response callback.
• Documented partial-drain behavior: delivered responses are not replayed or rolled back.
• Documented the single-driver contract: do not call insert, cancel, next, or close concurrently with drain.
• Added DocC guidance for bounded batches and long-running server loops.
• Added helper-level tests proving the drain scope wraps the full loop and cancellation stops before the next step.

Notes

Ticket sizing remains caller-owned in this PR. Automatic continuous-batch memory estimation is intentionally left for a future scheduler/container layer because it depends on cache topology, prompt shape, completionBatchSize, and admission policy.