feat: honour the boards a folder offers a core on

ROCKNIX writes some add_emu_core calls under a case on DEVICE, and the
rom folders file keeps that branch as the offer's `when` (uae4arm for
amiga, emulators/package.mk:188). A board's core list names such a core
when another folder offers it there, so --target placed its files in a
folder the board does not offer it for.

offer_boards() reads `when` and board_defaults against the platform's
targets, aliases included. A board build (generate_pack and generate
--manifest with --target, verify --target) carries the board in the
config (TARGET_KEY), and content_folder_tags leaves out the pairs the
board is not offered. A manifest for every board lists in
offered_boards each profile placed in such a folder, and install.py
--target drops its files there on the other boards.

Manifests: no file moves. The cores ROCKNIX offers under a device case
read no file beside the game today, so no manifest carries
offered_boards yet.

scripts/verify.py

@@ -46,6 +46,8 @@ from alternatives import (
     slot_set_keys,
 )
 from common import (
+    TARGET_KEY,
+    canonical_target_name,
     OUTER_ROOTS,
     _foreign_tree,
     _owner_tokens,
@@ -3302,6 +3304,11 @@ def _main(held: contextlib.ExitStack) -> None:
     for group_platforms, representative in groups:
         config = load_platform_config(representative, args.platforms_dir)
         tc = target_cores_cache.get(representative) if args.target else None
+        if tc is not None:
+            # A core a roms folder offers on other boards only is not placed
+            # there, as generate_pack builds the board's pack.
+            config = {**config, TARGET_KEY: canonical_target_name(
+                [representative], args.target, args.platforms_dir)}
         result = verify_platform(
             config,
             db,