Let authors place named magic items in treasure caches

[mmacy]

What

An authored treasure cache could carry coins, mundane equipment ids, and named valuables — but a specific magic item (the sword +1 in the captain's strongbox) was unplaceable at every layer: magic entered play only through generated hoards. This PR adds the authoring surface:

• FeatureSpec.magic_item_ids — an additive tuple of ids from the shipped magic-item catalog (see the magic item id index). Listing an id twice places two.
• instantiate_magic_item — the named-item half of generate_magic_item, factored out as a public function: no table selection, just the depth-first creation-detail resolution (the Magic Armour Type d8, charges, quantities, wish counts, scroll contents, the energy-drain total, sword sentience). generate_magic_item now calls it per row item, so the two paths cannot drift.
• Take-path instantiation — hand-placed magic items instantiate when the cache is emptied, details rolling on the treasure stream at the tier in force at that moment, exactly as a generated hoard's would. Authored content stays frozen; a save made before the take carries no undiscoverable referee facts.
• Validation — validate_adventure resolves the new ids against the shipped magic-item catalog (loaded internally; adventures bundle no magic items), a hard content error before play. validate_content_pack reports the same gap as the new pack.feature.unknown_magic_item finding.

Contracts

• No schema bump: additive field with an empty default; existing documents load unchanged.
• Determinism: a cache placing no magic items consumes no extra draws — the goldens are untouched. Placed items draw on TREASURE_STREAM in tuple order at take time.
• Layering: no new core→crawl imports; the take path in osrlib.crawl calls into osrlib.core.treasure.

Tests

• test_treasure_kernel.py::TestMagicItemInstantiation — details roll like a generated item's (charges 2d10, ammunition 3d10, generic armour's base d8), same seed same instance, unknown id/tier raise.
• test_exploration.py::TestAuthoredMagicItems — placed items land in packs unidentified with allocator ids, ride ItemAcquiredEvent, the cache empties once, and two seeded runs roll identical details.
• test_dungeon.py — a shipped magic id validates; an unknown one is a ContentValidationError naming the id.
• test_content_pack.py — the new finding code, with the known id passing silently.

Full suite: 1668 passed, 58 skipped. ruff format/check clean, mkdocs build -s clean.

https://claude.ai/code/session_01FAGD8y9X4XeCdZN1nuLJxc