Complete 26.3 parity acceptance and dual-version release workflow

ports/mc263/RELEASE_NOTES_26.3.0.3.md

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+# Rivencraft 26.3.0.3
+
+For **Minecraft 26.3**, **NeoForge 26.3.0.99** and **Java 25**.
+
+## Book preview exploration
+
+- The eye button now matches 1.21.1: it grabs the mouse for look-only exploration
+  while the destination view remains inside the book panel. Movement is disabled.
+- Right-click returns to the ordinary book controls without releasing the preview.
+  Books on stands retain their disabled exploration button.
+- Fixed the exit crash caused by a forced render and duplicate Reborn render-target
+  submission. Pending target submissions are reused until the renderer drains them.
+- Removed remaining Beta branding from the in-game mod name/description.
+
+## Port acceptance and maintenance
+
+- Completed dedicated-server restart/return-link acceptance for #100, following
+  the two-client matrix and its manual handoff. The player confirmed return travel.
+- Added changed-target build/smoke workflows, sequential test-server exclusion and
+  repeatable dual-version validation commands.
+- Hardened release uploads against mismatched filename, embedded version,
+  Minecraft/Java target and accidental test-fixture packaging. The 1.21.1 metadata
+  declares its required Dynamic Dimensions AR and Bad Packets dependencies.
+- Documented the feature parity review and ongoing independent support of 1.21.1.
+  Existing 1.21.1 code and published artifacts are unchanged.
+
+## Validation and known issues
+
+The player passed the live-view exploration/right-click/re-entry check and the
+dedicated-server Age restart/Linking Book return check. The expanded-catalog
+two-client matrix passed creation/catalog/travel/stale-ack/lifecycle checks;
+its final return step stalled and was completed by the player, not retried.
+Release-tooling checks pass. No optional mod or test fixture is bundled.
+
+**Known issue #116:** Static Sky can leak outside book previews with Reborn.
+It remains separately tracked. Other shared follow-ups remain open where needed.
+Create/Ponder is not ported to 26.3. Without Reborn, built-in animated previews
+and normal book/portal travel remain available. 1.21.1 saves are not imported.