initial: inventory-list-test v0.1.0 — 12 assertions

Covers engine spike (multi-slot children), add/remove/contains/count
round-trips, Loud-Error paths (no container block, no stack_mode,
stack_mode=stack), renderable-tag toggle via set_tag, and slot-naming
reconstruction from get_children (simulated post-reload state).

README.md

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+# lib-core.inventory-list-test
+
+Test suite for `lib-core.inventory-list` v0.1.0.
+
+**12 assertions:** 1 engine spike + 11 spec assertions.
+
+| # | Description |
+|---|---|
+| spike | `engine.entity` supports 3 children on distinct slot names |
+| 1 | `add` → `contains` true |
+| 2 | after `add`: `count == 1`, `contents` contains item |
+| 3 | two `add`s → `count == 2`, both in `contents`, distinct slot names |
+| 4 | `remove` → `contains` false, `count == 0` |
+| 5 | `add` to entity without container block → Loud-Error |
+| 6 | `add` entity without `stack_mode` → Loud-Error |
+| 7 | `add` item with `stack_mode="stack"` → Loud-Error |
+| 8 | `add` to non-container-template entity → Loud-Error |
+| 9 | `add` removes item from renderable index |
+| 10 | `remove` restores item to renderable index |
+| 11 | slot-naming reconstruction from `get_children` (simulated reload) |
+
+**Run:**
+```bash
+SPOREL_LIBS_DIR=~/Projects/Sporel/sporel-libs SPOREL_CI=1 SPOREL_LOG_STDERR=1 \
+  ./build/Sporel.exe --test-libs=lib-core.inventory-list-test 2>&1 | tail -30
+```