dm-system.md: fold the voice.md cross-reference into the paragraph so it
renders as part of the Cross-instance access explanation instead of an
orphaned bullet.
websocket.md: add 'host_unreachable' to the dm_call_undeliverable phase
union — stale since #32 was merged (docs drift noted in Task 8 review).
federation.md — new undeliverable bucket subsection with three-way
classification table, Path A/B semantics, and wire backward-compat note.
voice.md — no_recipient row in failure-surface table.
websocket.md — DmCallUndeliverableReason union updated to include no_recipient.
dm-system.md — cross-reference to voice.md for no_recipient reason.
Jannis Braun