[oglassdev]

Closes #120
Part of #115

Adds horse mode, the locked full-screen view from #114 ("Horse cursor"), ported from the prototype.

How it works

• View lock (Minestom): a new MinestomFullscreenHorse spawns two client-only entities. The first is an invisible bat at the eye, which the camera spectates. The second is an invisible, tamed, saddled horse with no AI, speed 0 and jump 0, which the player rides. The horse wears copper horse armour whose equippable asset is the pack's fullscreen/horse_cursor. On close: the camera returns to the player, the player dismounts, and both entities are destroyed.
• Cursor (pack): horse packs get a generated core/entity.vsh (vanilla 26.2 plus one branch), an equipment asset, and an armour texture: the signature texel at (0, 0), then the built-in cursor. Equipment layers (NO_OVERLAY with PER_FACE_LIGHTING) whose texel (0, 0) matches the signature recover yaw and pitch from the head-top corners. They draw that face as the cursor with the include's window_fs_cursor_at, at the cursor layer's depth, and hide every other vertex. window_fs_ndc and window_fs_depth are shared with text.vsh through one Rust snippet, and text.vsh stays byte-identical.
• Contract: fullscreen::horse holds the offset, signature and asset. The manifest/Kotlin FullscreenCanvasEntry gains an optional horse entry (offset, armour asset id), present only when a horse window exists.
• Runtime: removed the horse refusal. FullscreenDefinition.cursor is null in horse mode, so no text cursor run is sent.

Design choices

• Cursor and hit test agree: the client copies the rider's yaw and half their pitch onto the horse (AbstractHorse.getRiddenRotation, checked in 26.2 bytecode). The shader doubles the recovered pitch and calls the same window_fs_cursor_at as the text shader. The server maps the player's own rotation packets with FullscreenMapping, as in the unlocked mode. Hover stays server-driven: outlines are re-sent when the hovered control changes.
• Horse placement: the horse is at (0, −1.75, 3) from the eye instead of the prototype's (0, −1, 3). It stays in view at every FOV, so it is never frustum-culled, but it is below the camera's line of sight. Otherwise the client's pick ray (from the bat's eye) could hit the horse instead of the interaction entity: clicks would be order-dependent, or become misses with a cooldown.
• Spawn order: the bat and horse spawn on the tick after open, right after fake survival. A spectator client would draw the invisible horse as a translucent ghost. So the prototype's pack-wide "hide spectator ghost alpha" hack is not needed and not ported.
• HUD: the world-age gate stays. In 26.2 the contextual bar (here the horse's jump bar or the XP bar), the XP level, the selected item name and the crosshair still draw with a non-player camera. The max-health and absorption clamps are skipped in horse mode: hearts (the player's and the vehicle's) draw only for a player camera. The blank hotbar stays (the hand and item name). An empty action bar replaces vanilla's "Press Left Shift to Dismount" hint.
• Edge pull-back: kept unchanged. The cursor still clamps at the edges, so without pull-back an overshoot would have to be undone before the cursor moves. In this mode the correction only re-aims the horse: the camera is the bat, so the snap-back seen in the unlocked mode is not visible.

Files touched outside the new ones

MinestomFullscreen.kt (create the horse, lock it on the fake-survival tick, unlock it on restore, skip the heart clamp), FullscreenSession.kt (removed the horse check), FullscreenDefinition.kt (cursor is null in horse mode), FullscreenHost.kt/FullscreenView.kt (docs), FullscreenEntries.kt (HorseEntry), hud/renderer/fullscreen.rs (extracted the placement snippet), hud/mod.rs (horse_entity_shader), pipeline/fullscreen.rs, manifest, codegen, the fullscreen golden.

Checks

• just test-rust -p window-core fullscreen: 17 passed. New: armour texture, and horse-mode pipeline emission (shaders, equipment, texture, manifest round trip, pack_format 84 rejected).
• just gradle :runtime:test --tests '*Fullscreen*' and :minestom:test --tests '*Fullscreen*': pass. New: horse mode sends no cursor run and hit-tests the same rotation; horse lock, unlock and no heart clamp.
• just golden: only fullscreen changed. Three new files (entity.vsh, the equipment JSON, the armour PNG) plus debug.json and WindowFullscreenEntries.kt. text.vsh, the include and the HUD shaders are unchanged, so unlocked-only packs are byte-identical.
• glslang: entity.vsh with its includes inlined compiles for all 32 combinations of PER_FACE_LIGHTING, NO_OVERLAY, EMISSIVE, NO_CARDINAL_LIGHTING and APPLY_TEXTURE_MATRIX. A deliberately broken copy fails, so the check catches errors.
• just lint: clean.

Not verified in a real client

• The whole mode on this branch. The shader math is ported verbatim from the hand-tested prototype, but the new horse offset, fake survival (the prototype used real spectator mode) and the armour texture layout have not been run.
• The pitch is off for about half a second after opening, while the camera's eye height settles from the player's to the bat's (as in the prototype).
• The armour signature texel is reserved pack-wide.