docs: specify the Zakura p2p connection, discovery, and service-discovery stack

[evan-forbes]

Motivation

The Zakura (P2P v2, over iroh/QUIC) stack has a detailed spec for block-sync
congestion control (docs/specs/blocksync/congestion_control.md) but nothing
comparable for the layers underneath it. This adds a spec set for the parts of
the stack common to every service — how peers connect, how they find each
other, and how they learn which services each other speak — so the
invariants (bounded resources, authenticated/scoped identity, the connection
generation guard, per-IP admission, capability negotiation) are written down and
reviewable independently of the code.

No linked issue; documentation-only.

Solution

New docs/specs/p2p/, same house style as the block-sync spec (Overview →
Glossary → MUST/SHOULD → Edge cases and bounds → Defaults table):

• README.md — overview, an architecture diagram, and the shared vocabulary:
  the three service namespaces (stream kind / capability bit / service id), the
  stream-kinds and capability-bits tables, network/chain (cohort) scoping, and
  the wire magics.
• connections.md — the iroh/QUIC transport: connection lifecycle, the
  generation-guarded per-peer registry and transcript-hash duplicate
  resolution, framing/streams, timeouts/keepalive/freshness reaping,
  backpressure, rate limiting & DoS bounds, and identity/auth.
• peer_discovery.md — the address book and its four sources, the
  signed-record discovery protocol, the dial-slot-limit formula, the two backoff
  mechanisms, connection targets, and candidate lifecycle/scoring/eviction.
• service_discovery.md — capability negotiation (handshake intersection),
  stream-kind routing/admission, how each service acquires and admits peers, and
  the symmetric block-sync collision tiebreak.

The spec reflects current behavior including the generation-guarded connection
registration (WS-1) and the native per-IP connection default of 3 (WS-4); WS-5
(block-sync parking) is flagged as out-of-scope future work where relevant.

Tests

Documentation only — no code paths changed.

• markdownlint-cli2 --config .trunk/configs/.markdownlint.yaml docs/specs/p2p/*.md → 0 errors
• codespell docs/specs/p2p/*.md → clean

Specifications & References

• Sibling spec: docs/specs/blocksync/congestion_control.md
• Related: book/src/dev/private-zakura-network.md (dev-network cohort scoping)

Follow-up Work

• Optionally wire these into the mdBook nav (book/src/SUMMARY.md); the existing
  block-sync spec is likewise a standalone reference doc, so they're left out of
  the book nav for now.

AI Disclosure

• AI tools were used: Claude Code — explored the zebra-network/src/zakura
  transport/discovery/handshake modules and drafted the specs from that reading;
  all claims were cross-checked against the source. Reviewed by the author.

PR Checklist

• The PR title follows conventional commits format.
• The documentation is up to date (this PR is documentation).