feat: give each set its requirement and one pick

Three changes to the search-list model, one commit because the readers
and the profiles must move together for the schema to hold.

Set requirement in a groups map. pom2 falls back on apple2.rom for the
][ and the ][+, and on apple2e.rom for the //e it boots and for the
unenhanced //e: one required on the file could not say both. A profile
now declares groups: {name: {required}}, a member writes no required
of its own (schema and validate_schemas refuse one) and load_emulator_
profiles derives it, true when one of its sets is. group takes a name,
a list, or a map from set to rank where the rank differs per set. pom2
is grouped from SystemProfile.cpp and MainWindow.cpp:246-296 at its pin.

One member per required set under --required-only. The pick is the
first the code reads among the held members the region filter leaves,
by rank then declaration order; a set the platform declares a member
of is carried by that declaration. The emulator pack, verify
--emulator, the platform pack, its manifest and verify --platform read
alternatives.required_only_spares or core_file_spares.

variant_group is now selection_slot: files a core chooses between by
a rule other than first found (disc territory for PicoDrive and
Pizza Boy SC, the o2em_bios option, a WebAssembly probe in SWF
Player). Where the code takes any one, the field became a group:
ARMSX1 (ArmSxBackend.java:149-157, name order), ePSXe (load_bios
0x433540-0x433621 of the 2.0.5 Linux x64 binary, one configured path,
HLE when BiosHLE is set), FPse and FPse-NG, DXX-Rebirth's HOGs
(inferno.cpp:472-512). odin and zesarux carried it with no effect.
slot.py compares ranks only between candidates that share a set.

scripts/cross_reference.py

@@ -23,7 +23,7 @@ from collections import Counter
 from pathlib import Path
 
 sys.path.insert(0, os.path.dirname(__file__))
-from alternatives import set_keys, settle_sets
+from alternatives import set_keys, set_requirements, settle_sets
 from common import (
     platform_arg,
     ProfileSelectionError,
@@ -697,7 +697,7 @@ def slot_key(f: dict, standalone: bool = False) -> tuple:
         f.get("archive"),
         system_dir_path(f, where or fname),
         f.get("system"),
-        f.get("variant_group"),
+        f.get("selection_slot"),
         f.get("mode", "both"),
         read_from_save_dir(f, standalone),
     )
@@ -886,7 +886,7 @@ def _cross_reference_profile(
             archive_key = (
                 archive,
                 f.get("system"),
-                f.get("variant_group"),
+                f.get("selection_slot"),
                 f.get("mode", "both"),
             )
             if archive_key not in archive_gaps:
@@ -969,12 +969,17 @@ def _cross_reference_profile(
     # nobody holds is one gap under its first member (alternatives.py), as
     # verify and the packs count it.
     grouped = [r for r in set_records if r[0]]
-    excused, standing = settle_sets([(keys, f, fills) for keys, f, fills, _l, _r in grouped])
+    requirements = set_requirements(profile)
+    excused, standing = settle_sets(
+        [(keys, f, fills) for keys, f, fills, _l, _r in grouped],
+        lambda key: requirements.get(key[1], True),
+    )
     for at, (_keys, member, _fills, rows, set_row) in enumerate(grouped):
         if set_row is None or rows is None:
             continue
         if at in standing:
-            set_row["alternatives"] = standing[at]
+            set_row["alternatives"] = standing[at].others
+            set_row["required"] = standing[at].required
         if at in excused:
             rows.remove(set_row)
             fates[position[id(member)]] = "other_versions"