treat biological actuators as normal budget-competing mass, fix explore-panel visibility

Biological actuators (Human Muscle, Animal Traction) were special-cased
out of the mass optimizer entirely: a fixed 70kg reference used only in
the power formula, excluded from the platform's mass budget and from
the explore-panel sliders. That made "bigger operator" or "more
operators" inexpressible, and required a power_mass/denom_offset
parameter pair throughout the physics code solely to keep this one
case's numerator mass separate from its budget mass.

Operator mass is now a normal, budget-competing, structurally-carried
variable sized by the same joint optimizer as any mechanical actuator,
with BIOLOGICAL_OPERATOR_MASS_KG reinterpreted as a floor (at least one
real operator) rather than a fixed value -- the explore slider now
reads as "how many/how large are the operators." Since every remaining
case set power_mass == actuator_mass and denom_offset == 0.0 anyway,
those parameters were entirely vestigial once biological's special
case was gone, so _raw_physics_from_masses drops them.

Also: the explore section was gated on `explore_result is not none`,
so combos with no free mass to explore (radiation-pressure sails, or
previously biological) showed nothing at all instead of the existing
explanatory message. Gated on `scores` instead, so the section always
renders and the message inside `_explore_result.html` is reachable.

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

src/physcom_web/templates/results/_explore_result.html

@@ -1,7 +1,8 @@
 {% if explore_result is none %}
 <p class="empty">No free mass allocation to explore for this combination — its
-actuator's mass isn't a design choice (a physiological or footprint-derived
-quantity), or the platform has no declared mass ceiling to bound the sliders.</p>
+actuator's mass isn't a design choice (a footprint-derived quantity, like a
+radiation-pressure sail), or the platform has no declared mass ceiling to
+bound the sliders.</p>
 {% else %}
 {% set r = explore_result %}
 <div class="optimize-summary">

src/physcom_web/templates/results/detail.html

@@ -129,7 +129,7 @@
 </div>
 {% endif %}
 
-{% if explore_result is not none %}
+{% if scores %}
 <h2>Explore: Scale the Build</h2>
 <p class="subtitle">
   Purely exploratory — nothing here is saved. Drag a slider to pick a
@@ -141,6 +141,7 @@
   or merely functional one.
 </p>
 <div class="card">
+  {% if explore_result is not none %}
   {% set r = explore_result %}
   <form id="explore-form"
         hx-post="{{ url_for('results.explore', domain_name=domain.name, combo_id=combo.id) }}"
@@ -154,7 +155,7 @@
       <output id="out_platform_mass">{{ "%.1f"|format(r.platform_mass) }}kg</output>
     </div>
     <div class="weight-slider-row">
-      <label for="actuator_mass">actuator (motor size)</label>
+      <label for="actuator_mass">actuator (motor size, or operator count/size for muscle power)</label>
       <input type="range" min="{{ r.actuator_min }}" max="{{ r.actuator_slider_max }}" step="0.1"
              id="actuator_mass" name="actuator_mass" value="{{ r.actuator_mass }}"
              oninput="document.getElementById('out_actuator_mass').textContent = (+this.value).toFixed(1) + 'kg'">
@@ -168,6 +169,7 @@
       <output id="out_storage_mass">{{ "%.1f"|format(r.storage_mass) }}kg</output>
     </div>
   </form>
+  {% endif %}
   <div id="explore-result">
     {% include "results/_explore_result.html" %}
   </div>