Fix denial of service, information disclosure and TLS identity bugs

An adversarial review of the request path turned up nine issues, all
reproduced against a running server before and after the fix.

The most serious was a permanent denial of service. The in-flight request
counter was decremented on paths that never incremented it; being unsigned
it wrapped to usize::MAX, after which every request was answered with
"41 Server unavailable" until the process restarted. A client that completed
the TLS handshake and disconnected without sending a request was enough --
a port scan, a health check, a cancelled page load. The counter is replaced
by a semaphore permit released on every exit path.

Two further remote denials of service: an error from accept() propagated out
of main and ended the process, so exhausting the descriptor limit killed the
server; and responses were read whole into memory, so cost scaled with
concurrent requests times file size. Accept errors are now logged and retried
after a backoff, and responses stream in 64 KiB chunks capped at 64 MiB.
Peers that open a socket and never send a ClientHello are bounded by a 10s
handshake timeout and a connection cap (max_connections, default 512).

Every virtual host was served whichever certificate came first in a HashMap,
which varies per process, so a host's certificate changed between restarts.
Because Gemini clients pin certificates on first use, this trained users to
dismiss the mismatch warning that would otherwise reveal interception.
Certificates are now selected by SNI with a deterministic fallback, so
unknown hosts still complete a handshake and receive 53.

Dotfiles inside a content root were public, exposing .git/config and any
credentials in it for capsule roots that are git working copies. Path
resolution is now in-tree: reject non-plain components, then compare
canonical prefixes. This replaces path-security, an unaudited micro-crate in
the security boundary that also rejected legitimate filenames containing
'%', '~' or '$'.

rustls moves 0.21 -> 0.23; the 0.21 branch is end of life. TLS 1.2 and 1.3
only, as before.

Malformed requests logged attacker-controlled text at ERROR on every request,
letting a client drive log volume; client-caused conditions now log at debug,
truncated. Hostname routing accepts the authority forms the specification
permits (mixed case, explicit port, userinfo, trailing dot), which previously
returned 53. Per-host port and log_level were accepted and silently ignored,
and now warn at startup.

Adds tests/connection_lifecycle.rs covering the counter underflow, the
oversized-response refusal, hidden files and authority normalization. Fixes
the test client, which sent host:port as SNI. 42 -> 56 tests.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

dist/config.toml

@@ -28,21 +28,26 @@ port = 1965
 
 # Request limiting
 #
-# max_concurrent_requests: Maximum number of simultaneous connections
-#   - Prevents server overload and DoS attacks
-#   - Set to 0 to disable limiting (not recommended)
+# max_concurrent_requests: Maximum number of requests processed at once
+#   - Requests above the limit are answered with "41 Server unavailable"
+#   - Must be between 1 and 1000000; the server will not start on 0
 #   - Typical values: 100-10000 depending on server capacity
 max_concurrent_requests = 1000
 
+# max_connections: Maximum number of connections handled at once
+#   - Connections above the limit are dropped immediately, which keeps the
+#     listener responsive under a flood of half-open connections
+#   - Keep this below the process file-descriptor limit (LimitNOFILE in the
+#     systemd unit, or `ulimit -n`)
+#   - Must be between 1 and 1000000; defaults to 512 if unset
+max_connections = 512
+
 # Logging configuration
 #
-# log_level: Controls how much information is logged
-#   - "error": Only errors that prevent normal operation
-#   - "warn": Errors plus warnings about unusual conditions
-#   - "info": General operational information (recommended)
-#   - "debug": Detailed debugging information
-#   - "trace": Very verbose debugging (use only for troubleshooting)
-log_level = "info"
+# Logging is controlled by the RUST_LOG environment variable, not this file.
+# In the systemd unit, set for example:
+#   Environment=RUST_LOG=info
+# Levels, least to most verbose: error, warn, info, debug, trace
 
 # Host configuration
 # Each hostname needs its own section with root, cert, and key settings