Add project licensing and provenance documentation

THIRD_PARTY_NOTICES.md

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+# TurtleMail third-party notices
+
+Audit date: 2026-08-31
+
+This file records known upstream sources, inherited code, assets, compatibility
+references, and unresolved licensing for the Dusk-92 TurtleMail fork.
+
+Existing source comments, Git history, upstream repository metadata, and README
+credits remain part of the provenance trail.
+
+## Historical source: shirsig/Mail
+
+The original known source repository is:
+
+- https://github.com/shirsig/Mail
+
+It provides the earlier Vanilla WoW mail addon from which TurtleMail ultimately
+descends.
+
+No explicit project-wide LICENSE file was present in that repository during
+this audit.
+
+## Immediate upstream: sica42/TurtleMail
+
+The immediate upstream repository is:
+
+- https://github.com/sica42/TurtleMail
+
+That repository is itself a fork of `shirsig/Mail`.
+
+No explicit project-wide LICENSE file was present in the immediate upstream
+repository during this audit.
+
+Accordingly, this fork does **not** claim that inherited TurtleMail or Mail code
+is MIT, GPL, public domain, or otherwise freely relicensed. The root `LICENSE`
+notice is intentionally limited in scope and does not override upstream rights.
+
+## Dusk-92 compatibility layer
+
+The current fork adds and maintains Turtle WoW / Octo-like compatibility and
+safety work.
+
+Notably:
+
+- `TurtleMailFix.lua` is maintained in this fork as an isolated compatibility
+  layer.
+- `AUDIT.md` documents the compatibility/stability audit.
+- `TurtleMail.toc` and `README.md` include fork-specific maintenance and
+  compatibility changes.
+
+Historical Git commits remain the authoritative record for individual changes.
+
+## Unchanged immediate-upstream files
+
+At the time of this audit, the following runtime files in the Dusk-92 fork were
+byte-identical at Git blob level to `sica42/TurtleMail`:
+
+- `Calendar.lua`
+- `TurtleMail.lua`
+- `TurtleMail.xml`
+- `localization.lua`
+- `localization.de.lua`
+- `localization.es.lua`
+- `localization.fr.lua`
+- `localization.ru.lua`
+- `TurtleMail-AH.blp`
+- `TurtleMail-RetArrow.blp`
+- `TurtleMail-DownArrow.tga`
+
+This establishes immediate provenance, not a new license grant.
+
+## Calendar
+
+`Calendar.lua` is inherited unchanged from `sica42/TurtleMail`.
+
+The immediate upstream history shows the calendar as part of the 2025 logging
+work, with a later calendar dropdown bug fix by Sica.
+
+No separate license for that file was identified, so it remains documented as
+upstream-derived material under unresolved project-wide licensing.
+
+## Visual assets
+
+The three bundled visual assets are tracked in
+`Docs/ASSET_PROVENANCE.md`.
+
+Their immediate upstream source is verified, but their ultimate artistic source
+or underlying asset license was not established during this audit.
+
+No additional ownership or relicensing claim is made.
+
+## pfUI integration
+
+TurtleMail includes optional compatibility/integration behavior for pfUI.
+
+Compatibility or API integration does not imply that pfUI is bundled, nor does
+it imply affiliation or endorsement.
+
+## Turtle WoW / Octo-like compatibility
+
+This fork targets Turtle WoW 1.18.x and Octo-like Vanilla environments.
+
+Compatibility, naming, testing, or behavioral reference does not create an
+affiliation, endorsement, partnership, or ownership relationship with those
+projects or their maintainers.
+
+## Project identity and trademarks
+
+Canonical maintained fork:
+
+- https://github.com/Dusk-92/TurtleMail
+
+World of Warcraft, Warcraft, Blizzard Entertainment, and associated names,
+marks, artwork, and game assets remain the property of their respective rights
+holders.
+
+See `PROJECT_IDENTITY.md`.
+
+## Preservation rule
+
+Do not remove historical attribution, source comments, upstream references, or
+provenance records merely because inherited code is later modified.
+
+When replacing or substantially rewriting inherited material, update the
+provenance record rather than erasing the historical chain.