Add selector nomenclature for conditional field visibility

A selector is a nomenclature whose value decides which other nomenclatures
appear beneath it: picking "Ship" reveals the ship fields, "Warehouse"
reveals the warehouse ones. Previously every vehicle nomenclature rendered
at once, so the operator had to know which were relevant and a vehicle
could silently carry both a ship and a warehouse.

Visibility is one rule - a nomenclature is hidden if some selector option
reveals it and that selector's value is not the revealing option - kept in
nomenclatures/visibility.py and mirrored in services/nomenclatureVisibility.js.
A nomenclature no selector references always shows, so installations without
selectors are unchanged.

The serializer, not the UI, enforces the invariant: a value for a hidden
nomenclature is rejected, as is a selector value that matches no option.
The frontend clearing values on switch and pruning hidden keys before save
is convenience on top of that.

Selector values join lookups in the quick-fill combination projection, so
applying a saved combination sets the source and its revealed fields in one
click rather than collapsing "Ship + cargo" and "Warehouse + cargo" into
one indistinguishable row.

Configuration rejects nesting (a selector revealing another selector) and
double-reveal (one nomenclature revealed by two selectors, leaving its
visibility ambiguous). The admin UI filters both out of the reveal list as
well, so an invalid configuration cannot be built in the first place.

Note: introducing a selector over existing data hides values already
entered - vehicles predating the selector have no selector key, so anything
it reveals becomes invisible until a source is chosen. The values remain
stored. Worth a data check before configuring the first selector.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

backend/nomenclatures/visibility.py

@@ -0,0 +1,48 @@
+"""The one rule deciding which nomenclatures are visible for a given extra_data.
+
+A nomenclature is hidden if it is revealed by some selector option and that
+selector's current value is not the option revealing it. Anything no selector
+references is always visible, so installations without selectors are unchanged.
+
+Mirrored in frontend/src/utils/nomenclatureVisibility.js - keep the two in step.
+"""
+
+from .models import Nomenclature, SelectorOption
+
+
+def build_reveal_map(nomenclatures=None):
+    """{revealed_code: {selector_code: {value, ...}}} for the given nomenclatures."""
+    options = (
+        SelectorOption.objects
+        .select_related('selector')
+        .prefetch_related('reveals')
+    )
+    if nomenclatures is not None:
+        codes = {n.code for n in nomenclatures}
+        options = [o for o in options if o.selector.code in codes]
+
+    reveal_map = {}
+    for option in options:
+        for revealed in option.reveals.all():
+            reveal_map.setdefault(revealed.code, {}).setdefault(
+                option.selector.code, set()
+            ).add(option.value)
+    return reveal_map
+
+
+def is_visible(code, extra_data, reveal_map):
+    """True if the nomenclature `code` should be shown/accepted for this data."""
+    controllers = reveal_map.get(code)
+    if not controllers:
+        return True
+    for selector_code, revealing_values in controllers.items():
+        if extra_data.get(selector_code) in revealing_values:
+            return True
+    return False
+
+
+def visible_codes(codes, extra_data, reveal_map=None):
+    """Filter `codes` down to those visible for `extra_data`."""
+    if reveal_map is None:
+        reveal_map = build_reveal_map()
+    return [c for c in codes if is_visible(c, extra_data or {}, reveal_map)]