Chat file attachments, stage 1: protocol + data model + pure seams

First slice of in-chat file/photo sharing (dedicated file plane, images
inline + file chips, session-only). This stage adds the wire types and
the pure, unit-tested logic; no transport or UI yet.

- protocol: new FILES_ALPN / FILES_PROTO (peerspeak/files/1) for the
  dedicated file-transfer plane. Bump GOSSIP_PROTO 1->2 + sig domain v2
  (Chat gained an attachment field, so cross-version peers fail fast
  rather than half-work) and Cargo 0.2.0 -> 0.3.0 per VERSIONING.md.
  BREAKING wire change: all peers must run >= 0.3.0.
- new src/files.rs: ChatAttachment descriptor (name/size/kind/id; bytes
  travel off-gossip), AttachmentKind, plus pure seams — sanitize_filename
  (path-traversal/control-char/length-safe), size_within_cap, image
  magic-byte sniffing + defensive limited decode (decode-bomb guard),
  32-byte request parsing, human_size. 13 unit tests.
- GossipMessage::Chat and RoomEvent::ChatMessage carry an optional
  ChatAttachment; send_chat takes Option<ChatAttachment>. Untrusted
  inbound descriptors are filename-sanitized + size-validated on ingest.
  serde(default) keeps the field forward-compatible at the JSON layer;
  +round-trip and pre-v2 back-compat tests.

The attachment id is a random 32-byte handle (rand, already a dep), not
a content hash — the fetch is authenticated + encrypted + member-gated,
so no crypto-hash dep is needed.

349 lib tests pass, clippy clean.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Cargo.toml

@@ -1,6 +1,6 @@
 [package]
 name = "peerspeak"
-version = "0.2.0"
+version = "0.3.0"
 edition = "2024"
 # Application crate, not a crates.io library — refuse `cargo publish` and let
 # cargo-deny's [licenses.private] skip the missing-license check.