fix: seed port allocations for every command, not just up and tasks run

[aostanin]

Problem

processes.<name>.ports.<port>.value resolves to the declared base port in every
command except devenv up and devenv tasks run, even while a process manager is
running with a different allocation. Env vars derived from it are wrong too.

{ config, pkgs, ... }:

{
  processes.server = {
    ports.http.allocate = 8080;
    exec = "${pkgs.python3}/bin/python -m http.server ${toString config.processes.server.ports.http.value}";
  };

  env.SERVER_PORT = config.processes.server.ports.http.value;
}

Run it in two clones of the project. The first takes the base port:

~/project $ devenv up -d
~/project $ devenv processes list
server    ready restarts: 0 ports: http:8080

The second allocates around it, but its shell reports the first clone's port:

~/project2 $ devenv up -d
~/project2 $ devenv processes list
server    ready restarts: 0 ports: http:8081

~/project2 $ devenv shell -- printenv SERVER_PORT
8080

Root cause

reserve_running_ports() seeds the allocator from the running native manager,
but is only called from up() and tasks_run(). shell (and therefore
devenv hook, which spawns devenv shell) and direnv-export never seed, so
allocatePort is not registered and .value falls back to allocate.

This is a regression of the #2710 fix rather than a gap in it.
88ac631
seeded inside assemble(), which every evaluating command went through;
d0f9c588
("split apart and remove assemble") kept the call only in up() and
tasks_run(). The example above reports the allocated port at commit
6792fc1e
and the base port at commit
d0f9c588,
so that is the commit where it stopped working.

Fix

Call reserve_running_ports() once after Devenv::new in run_backend, before
anything evaluates. Best-effort: it only seeds when a live-PID manager answers,
so the gating added in #3024 still applies. The socket query gets a 2s timeout,
since it now runs before every command and a socket that accepts but never
answers must not hang the CLI.

The existing calls in up() and tasks_run() are left in place — they are
redundant for the CLI but still cover direct use of those public methods.

Seeding still requires a live-PID manager, so a command run during a manager's
startup window — between its processes binding their ports and the PID file
being written after the readiness probes — resolves the base port as before.
tests/process-port-allocation-two-repos waits for the PID file for that
reason. Closing that window is a separate change.

Validation

• devenv-run-tests run --only process-port-allocation-two-repos tests, extended
  with a devenv shell assertion in the second project: fails on v2.2.1, passes
  with the fix
• devenv-run-tests run --only 'process-*' --only 'tasks-*' tests: 18 passed
• Two concurrent projects on x86_64-linux and aarch64-darwin: devenv shell,
  direnv-export and devenv processes list agree on the allocated port