Add code and asset provenance records

Docs/ASSET_PROVENANCE.md

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+# TurtleMail asset provenance
+
+Audit date: 2026-08-31
+
+No visual asset was modified during this documentation pass.
+
+A software or repository-level notice does not by itself establish ownership or
+relicensing authority for every visual asset. This file records what can be
+verified without making broader ownership claims.
+
+## Bundled visual assets
+
+| File | Git blob SHA-1 | Immediate upstream | Status |
+| --- | --- | --- | --- |
+| `TurtleMail-AH.blp` | `29727e7334dac8891f1483e2a07f8e53d0bae3d5` | `sica42/TurtleMail` | exact upstream Git blob match |
+| `TurtleMail-RetArrow.blp` | `ac2279ad04168c03e12d693e0056da843d2f3079` | `sica42/TurtleMail` | exact upstream Git blob match |
+| `TurtleMail-DownArrow.tga` | `fb6c3b2ca14cc6c4f612711a62184c270e329d3f` | `sica42/TurtleMail` | exact upstream Git blob match |
+
+## Immediate upstream history
+
+`TurtleMail-AH.blp` and `TurtleMail-RetArrow.blp` were introduced in
+`sica42/TurtleMail` by commit:
+
+`814a322b87d3262181e78bbdd1f2e6418ac14f8d`  
+"Add icons for returned & AH mails" — 2025-03-17
+
+`TurtleMail-DownArrow.tga` appears in the upstream logging work including
+commit:
+
+`91142ac27c8fabd651b8e8db6c3d30baa31a604e`  
+"New logging feature" — 2025-03-23
+
+## Provenance status
+
+The immediate source is verified. The ultimate creator/source and any separate
+underlying asset license were not established during this audit.
+
+Therefore:
+
+- no claim is made that Dusk-92 created these assets;
+- no claim is made that sica42 necessarily owned every underlying visual
+  element;
+- no additional license is asserted for these assets;
+- the files are retained with documented provenance.
+
+## Screenshots
+
+The README references externally hosted screenshots. They are not bundled as
+repository files.
+
+## Trademark and game-asset boundary
+
+World of Warcraft, Warcraft, Blizzard Entertainment, and associated names,
+marks, artwork, and game assets remain the property of their respective rights
+holders.