cmd/tailcat: add forward subcommand

[Audi-dask]

What does this change do?

Add a tailcat forward subcommand for exposing TCP ports served by a
Tailcat server as ordinary local TCP listeners.

This is useful for browsers, database clients, IDEs, and other
applications that only support regular host:port connections and do
not provide Tailcat, SOCKS, or stdio proxy integration.

The <addrblob> argument is the address blob printed by tailcat serve,
typically a value beginning with tc.

Related to #14.

Examples

Start a Tailcat server:

tailcat serve 8080
# Server listening with new address: tcXXXXXXXXX

Forward the served port to the same local port:

tailcat forward tcXXXXXXXXX 8080

This forwards:

127.0.0.1:8080 -> Tailcat -> remote localhost:8080

Use a different local port:

tailcat forward tcXXXXXXXXX 18080:8080

Forward multiple ports:

tailcat serve 3306,6379
# Server listening with new address: tcXXXXXXXXX

tailcat forward tcXXXXXXXXX 3306:3306 6379:6379

Bind the local listener to a specific address:

tailcat forward --bind=192.168.1.100 tcXXXXXXXXX 18080:8080

Use --bind=0.0.0.0 only when intentionally allowing connections
from other network interfaces:

tailcat forward --bind=0.0.0.0 tcXXXXXXXXX 18080:8080

Design

• Reuses one Tailcat client for all forwarded ports.
• Uses the existing Client.DialTCPPort API.
• Uses the existing ProxyConns helper.
• Supports both <remote> and <local>:<remote> mappings.
• Supports multiple local-to-remote port mappings.
• Binds to 127.0.0.1 by default.
• Supports an explicit local bind address with --bind.
• Does not change the Tailcat protocol or server behavior.
• Does not introduce a new dependency.

Testing

• Added unit tests for local and remote port mapping parsing.
• Added CLI command tree coverage for the forward subcommand.
• Added an end-to-end TCP forwarding test using the existing local DERP,
  STUN, and Tailcat server test environment.
• The end-to-end test waits for the local listener by retrying TCP
  connections instead of parsing diagnostic logs.

Tested with:

GOMAXPROCS=2 go test ./...