Serve modules over persistent connections

pure-serve replaces the module mode of server.c, which goes back to what
it was. It keeps the document contract, the runtime core (a work slot per
connection, the payload budget, core-owned ingress, borrowed PUT replies)
and the group commits, and adds what server.c lacked: HTTP/1.1 persistent
connections with in-order pipelining, an edge-triggered epoll owner
thread, up to 1,024 connections allocated as they arrive and reused, one
writev per answer, and documents opened through a store handle.

On the Pi 5 with one thread it serves 106k small keep-alive reads per
second (axum on three cores: 126k) at the same p99, 1,741 1 MiB reads
(axum: 1,370), and 1,126 durable writes from 64 connections (crash-safe
axum: 372), in 7.3 MiB against axum's 17-20 MiB. Note 075 has the table.

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