shutdown-GPF fix (exit_group after cleanup) + docs curation/musl verdict

recovery hardening: validate seq chain in scanForLastSeq

tier 1 postgres workload reduction: host_id cache + index + count short-circuit

v0.0.2 — src reorg: main.zig + internal/

pure move + import rewiring, no behavior change. matches build.zig.zon .version.

v0.0.1 — baseline before subsystem reorg

flat src root with util/ and atproto/ extracted. matches build.zig.zon .version.