add speed as a derived metric, fix aerodynamic drag gap it exposed, expand urban_commuting

target_velocity was only ever a platform-declared input used to size the
actuator -- an achieved-speed OUTPUT never existed anywhere, even though
trip time clearly matters for a domain like urban commuting. Added
"speed" as a genuine derived metric: achieved steady-state cruise speed
computed from the build's own power_density and the medium's resistance,
the same way power_density/range_fuel/cost_efficiency are already
outputs of a build rather than inputs to it.

That immediately surfaced a known, previously-deferred gap: the
resistance model was mass-proportional only (rolling resistance), with
no velocity-squared aerodynamic drag term, so inverting power/resistance
for speed had no ceiling at all -- light vehicles were "achieving"
thousands of m/s. Added DRAG_POWER_COEFF_BY_MEDIUM (ground only, a
car-like reference cross-section) and a closed-form cubic solve
(_solve_achievable_speed_mps, via Cardano's formula, no iteration) for
the achieved speed where propulsive power balances resistance + drag.
Reused the same effective (drag-inclusive) resistance for range_fuel and
cost_efficiency's operating-cost term, since they're the same physical
quantity (energy spent per meter) evaluated at the build's actual speed.

This also closes the range-overestimation bug flagged much earlier
against combo #876 (a real e-bike): range dropped from ~1,032km to
~53km, right in the ~50-80km realistic e-bike range that was the
original target. Air and water media are unchanged (air's L/D-based
cruise model doesn't have this problem; water hull drag needs its own
treatment, not a car's frontal area -- left as a known remaining gap).

Also added cargo_capacity_kg to urban_commuting (whether a commute
vehicle can carry groceries/passengers/gear matters as much as the
metrics already scored there) and renormalized weights across the now
five metrics.

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

src/physcom/seed/transport_example.py

@@ -741,9 +741,16 @@ URBAN_COMMUTING = Domain(
         # this project hasn't done -- not scored anywhere for now rather than
         # pretend a quick formula or an equally uninformed LLM guess settles it.
         # Weights renormalized to sum to 1.0 across the remaining metrics.
-        MetricBound("power_density", weight=0.4167, norm_min=1, norm_max=2000, unit="W/kg"),
-        MetricBound("cost_efficiency", weight=0.4167, norm_min=1e-5, norm_max=2e-3, unit="$/m", lower_is_better=True),
-        MetricBound("range_fuel", weight=0.1666, norm_min=5000, norm_max=500000, unit="m"),
+        # speed and cargo_capacity_kg added -- a commute's actual travel
+        # time and whether the vehicle can carry groceries/passengers/gear
+        # both matter as much as raw power_density did on their own; speed
+        # is a genuine build OUTPUT (see _raw_physics_from_masses), not a
+        # platform-declared constant.
+        MetricBound("power_density", weight=0.25, norm_min=1, norm_max=2000, unit="W/kg"),
+        MetricBound("cost_efficiency", weight=0.25, norm_min=1e-5, norm_max=2e-3, unit="$/m", lower_is_better=True),
+        MetricBound("speed", weight=0.25, norm_min=2, norm_max=30, unit="m/s"),
+        MetricBound("range_fuel", weight=0.10, norm_min=5000, norm_max=500000, unit="m"),
+        MetricBound("cargo_capacity_kg", weight=0.15, norm_min=1, norm_max=500, unit="kg"),
     ],
     constraints=[DomainConstraint("medium", ["ground", "air"])],
 )