docs: inventory ECS expected failures

Amp-Thread-ID: https://ampcode.com/threads/T-01a02705-02f3-70c9-ae5b-8ea6c2a3cac2

Author: ampagent

docs/architecture/ecs/ecs-expected-failure-inventory.md

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+# ECS expected-failure inventory
+
+This inventory covers the compatibility regressions marked with Vitest
+`.fails()` on the ECS migration stack and its reproduction PRs. A marker should
+be removed only after the same test passes as a plain `it()` test.
+
+## Activated by this stack
+
+Five reproduction tests now pass normally:
+
+| Source          | Tests | Resolution                                                                                                                                  |
+| --------------- | ----: | ------------------------------------------------------------------------------------------------------------------------------------------- |
+| #15592 / #15577 |     3 | Preserve the input-referenced link through deduplication, retain it through serialization, and identify the occupied target in the warning. |
+| #15595          |     2 | Rehydrate indexed plain-object input slots so spread copies retain live connectivity.                                                       |
+
+## Remaining expected failures
+
+Eight tests remain marked `.fails()`:
+
+| Area                                 | Tests | Required fix                                                                                                  |
+| ------------------------------------ | ----: | ------------------------------------------------------------------------------------------------------------- |
+| Legacy slot link creation            |     2 | Proxy additions through `input.link` and `output.links` into link-store registration, not only removals.      |
+| Input-slot realignment (#15581)      |     4 | Remove or relocate unmatched blockers in the same topology update so valid sibling moves land without errors. |
+| Enumerable `LLink` topology (#15594) |     2 | Expose topology fields as own enumerable properties so link spread copies preserve their endpoints.           |
+
+The markers live in:
+
+- `src/lib/litegraph/src/node/legacySlotLinkMutations.test.ts`
+- `src/lib/litegraph/src/LGraph.inputSlotRealign.test.ts`
+- `src/lib/litegraph/src/LLink.spreadCopy.test.ts`