* cli: add repo serve command wiring the xfer HTTP server

Repo.ServeHTTP has always been public, but nothing in the CLI called it, so a repo created or cloned with this tool could only ever be a clone leaf. Add `libfossil repo serve` as a thin adapter: open the repo, build the bind address, translate process interrupt into the context cancellation ServeHTTP already honors, and call it. No new server or shutdown logic.

Binds to 127.0.0.1:8080 by default; --addr overrides host and/or port. Verified against a stock fossil client (canonical binary, not just this library's own transport) cloning over an actual TCP round trip.

* cli: address review nits on serve interop test

Document why TestRepoServeCmdRun and TestRepoServeCmdCanonicalFossilClone are allowed to send a real SIGINT to the test process (no t.Parallel(), no competing signal handlers in package cli_test), so a future test in this package doesn't silently break that assumption.

Also log the canonical fossil binary path and version the interop test actually exercised. A skip and a pass read identically in a non-verbose CI log; if the apt install step that puts fossil on PATH is ever removed or reordered, this line is what would tell us the test stopped proving anything instead of just staying green. check-in: 7572f7d420 user: [email protected] tags: trunk