TS6 server-to-server linking

[kevinpostal]

TS6 server-to-server linking for Ergo, extracted from waveplate/ergo branch ts6-linking (commits 622bc84, dc5c31e, 41b3f27, 3491c03, 0b05e5e, author @waveplate, cherry-picked verbatim; merge commit cc472a9 skipped). Cherry-picks applied cleanly against current master with no conflicts.

Depends on #2456 (stacked; the first commit on this branch is that PR's commit).

What the change contains:

• irc/s2s_manager.go — link lifecycle: dial/listen, reconnect, routing graph, netsplit cleanup, multi-hop broadcast.
• irc/s2s_handlers.go — inbound TS6 command handlers (PASS, CAPAB, SERVER, SVINFO, SID, UID/EUID, SJOIN, TMODE, BMASK, TB/TOPIC, JOIN, PART, KICK, MODE, PRIVMSG/NOTICE, AWAY, KILL, ENCAP, PING/PONG, SQUIT) plus nickname/UID resolution and TS collision handling.
• irc/s2s_burst.go — outbound network-state burst (servers, users, channels, modes, bans, topics).
• irc/s2s_link.go, irc/s2s_types.go — connection I/O and shared types; small hooks in channel.go, client.go, client_lookup_set.go (UID-indexed client lookup), nickname.go, handlers.go, commands.go, help.go, modes.
• Config: server.sid (this server's 3-character SID) and a server.links map per peer (name, sid, hostname, port, tls, send-password, receive-password, auto-connect) — see the commented examples in default.yaml/traditional.yaml and the docs/MANUAL.md section added by 0b05e5e.
• distrib/docker-linking/ (from 41b3f27) — two-server linked docker deployment (compose.yaml, ircd-a.yaml, ircd-b.yaml, README.md) for manual testing.
• Tests: irc/s2s_test.go — unit + integration suite for handshake, burst, runtime propagation and splits.

Reviewer note — inbound handshake password check (irc/s2s_handlers.go, handlePass, quoted verbatim):

	// Password validation
	if link.inbound {
		// Find matching link config if present
		cfg := s2s.FindLinkConfig(sid, "")
		if cfg != nil && cfg.ReceivePassword != "" {
			if pass != cfg.ReceivePassword {
				return fmt.Errorf("invalid link password for SID %s", sid)
			}
			link.config = cfg
		}
	}

i.e. an inbound connection presenting an unknown SID — one with no matching entry in server.links (matched by SID via FindLinkConfig) — is accepted without any password check (only TS-version, SID-format and self-collision validation apply). Operators must define a links entry for every peer they intend to link with; otherwise anyone can introduce a server into the network. Flagging explicitly so reviewers see it.

Upstream policy note: docs/MANUAL.md currently states "Ergo does not currently support server-to-server linking (federation), meaning that all clients must connect to the same instance." This PR is therefore opened as a draft for discussion, not as a merge-ready proposal.

Testing: make test was run locally but does NOT fully pass on this branch; make irctest was NOT run (needs the irctest harness). Verbatim failure (go test -tags "i18n mysql postgresql sqlite" ./irc/, full package run reports "170 passed, 1 failed"):

--- FAIL: TestCanonicalizeMaskWildcard (0.00s)
    strings_test.go:193: expected <hebrew nick> to canonicalize to <hebrew nick>!*@*, instead
    strings_test.go:196: expected <hebrew nick> to produce error <nil>, instead Invalid character
    strings_test.go:193: expected <cyrillic nick>!Potato to canonicalize to <lowercased>!potato@*, instead
    strings_test.go:196: expected <cyrillic nick> to produce error <nil>, instead Invalid character
FAIL
FAIL	github.com/ergochat/ergo/irc	5.736s

(The literal non-ASCII nicks are elided here; they are Hebrew and Cyrillicnick cases gated on i18n.Enabled.) The failure is a test-isolation interaction, not a product-code regression: the new irc/s2s_test.go boots test servers with casemapping: ascii, which sets the package-global globalCasemappingSetting; since s2s_test.go sorts before strings_test.go", its tests run first and the global is still ASCII when TestCanonicalizeMaskWildcard's i18n-gated cases execute. Evidence: the test passes on pristine upstream master, and passes on this branch in isolation (go test ./irc/ -run TestCanonicalizeMaskWildcard`). Left unfixed deliberately — fixing it would mean editing the contributed test setup, which is the author's call.

Author credit: @waveplate.

[slingamn]

Please see #1532 for previous discussion of this issue. The short answer is that TS6 and similar are not a good fit for Ergo, because core Ergo logic (e.g. nickname ownership) needs to be CP rather than AP.

At a more immediate level, I do not see how this PR addresses availability of the datastore (in particular the business logic in irc/accounts.go) across multiple linked nodes.