VendingMachinePanel._machine_inventories is a static var that persists
across scene reloads. Added a clear() call in game_over_panel alongside
the existing KnowledgeDB and ChunkManager resets.

Closes #34

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… Mustard)

Adds three new tutorial-only items for Level -1, each mapped to a pool:
- Meat (BODY, COMMON, +5 BODY) ~67% spawn chance
- Vegetables (MIND, UNCOMMON, +5 MIND) ~27% spawn chance
- Mustard (NULL, RARE, +5 NULL) ~7% spawn chance

Only one spawns per run, selected by weighted rarity. Items use level=5
with the base class stat bonus system for a flat +5 boost. Each has
PSX-style pixel art textures and clearance-gated descriptions.

Also removes dead `starting_items` property from level_config.gd and
replaces the hardcoded debug_item spawn in the tutorial generator with
weighted rarity selection.

Note: The generator explicitly includes "level" in item_data because
WorldItem.to_dict() doesn't serialize level yet (tracked in #44).

Closes #38

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Smooth visual transitions between grid positions for player and entities
using the Anathema Interloper pattern: tree-level tweens on local position
with TRANS_SINE + EASE_OUT at 0.2s duration. This avoids camera jitter
that occurred with node-level tweens and manual lerp approaches.

- Player movement tweens via get_tree().create_tween() on position
- Entity billboard/health bar tweens match player timing
- Settings toggle: "Move Smoothing: ON/OFF" in pause menu
- snap_visual_position() bypasses tween for init/teleport/level loads
- Utilities.movement_smoothing static var controls the feature

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Adds automatic exploration that navigates toward unexplored tiles at
configurable speed, with intelligent stop conditions and settings UI.

Core implementation:
- AutoExploreState: new player input state with per-step re-pathing,
  camera rotation to face movement, auto-pickup of items along path
- ExplorationTracker autoload: BFS-based nearest unexplored tile finder
  with chunk-preference bias (finishes current chunk before crossing)
- Activated via Y key / Y button, cancelled by any input

Stop conditions (all configurable in settings):
- HP or sanity below threshold (default 50%)
- Enemy detected in perception range
- Item or vending machine detected (navigates adjacent then stops)
- Stairs detected (navigates adjacent then stops)
- Took damage since last step
- No unexplored tiles or path blocked

Settings panel:
- Speed slider (1-30 turns/sec, default 10)
- HP/Sanity threshold sliders
- Toggle buttons for enemies, items, damage, stairs
- Accessible via Auto-Explore Settings button in pause menu

Other changes:
- ExplorationTracker marks tiles explored each turn in PreTurnState
- HUD indicator shows "AUTO-EXPLORE" when active
- Frame-based guard prevents Y key from cancelling then re-triggering
- Explicit ui_accept mapping (Enter, Space, JOY_BUTTON_A) for start menu
- Auto-explore log coloring in game.gd

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Players need to keep clearance pace with corruption-scaled item levels.
The old curve (100 × n^1.5) made mid-game leveling too slow, locking
players out of item information.

- XP curve: BASE 100→75, EXPONENT 1.5→1.25 (Lv5: 1118→447, Lv10: 3162→1334)
- Chunk exploration XP: 10→20
- Kill XP: min 10→15, divisor 10→5 (weak enemies feel more rewarding)

Net effect: ~2-3x faster progression across the board.

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Items now spawn at higher levels based on corruption: each 0.25 corruption
gives +1 guaranteed bonus level, with fractional remainder as a probability
roll. This applies to natural spawns, pity spawns, and vending machine
offerings. Item combining is now additive (a Level 3 pickup + Level 2
equipped = Level 5). WorldItem serialization preserves level through
chunk load/unload cycles. Also halved corruption's impact on sanity drain
to keep high-corruption runs survivable.

- item_spawner: Add _roll_item_level() with CORRUPTION_PER_LEVEL_STEP=0.25
- item_spawner: Apply corruption-scaled levels to spawn and forced spawn
- vending_machine_panel: Scale offerings with corruption
- pickup_to_slot_action: Additive combining (level_up(item.level))
- item.gd: level_up() accepts amount parameter
- world_item: Serialize/restore level in to_dict()/from_dict()
- sanity_damage_action: Halve corruption multipliers (10→5, 6→3)

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Any item can become corrupted, gaining random debuffs that are "dark
mirrors" of positive item effects. Corrupted items default to OFF and
display [CORRUPT] prefix with glitch animation on ground sprites.

Corruption system:
- STAT debuffs reduce body/mind/null_stat (mirrors +N stat bonuses)
- PER_TURN debuffs drain HP/Sanity/corruption (mirrors regen items)
- ON_USE debuffs reduce damage and increase mana cost (mirrors attack bonuses)
- All scale with item_level * (1 + corruption), worsening over time
- Clearance-gated descriptions hide debuff details below item level

Item lifecycle changes:
- on_enable/on_disable now toggle ALL effects (stat bonus + corruption)
- clamp_resources() ensures current HP/Sanity respect new max values
- starts_enabled=false items correctly start with no active effects
- Combining corrupted items merges debuff arrays

Spawning and display:
- Corruption-scaled spawn chance with bonus corrupted item spawns
- Glitch shader (chromatic aberration, scanlines, vertex wobble)
- Tween-based modulate animation for corrupted ground sprites
- Vending machines can offer corrupted items
- Corrupted items show distinct color in all UI panels

Wired-up on-use effects:
- damage_penalty multiplier applied during attack building
- mana_tax multiplier applied during attack building
- corruption_spike triggers on successful pool attack hit

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Root cause: find_nearest_unexplored() ran a full BFS traversal of the
entire walkable graph every auto-explore step (~10x/sec). As the explored
area grew, BFS became increasingly expensive — most noticeably after the
first vending machine detour expanded the explored region significantly.

Fix: Cache the BFS target and only recompute when the cached target tile
becomes explored (player reached it). Between steps the target doesn't
change, so this eliminates redundant O(explored_area) BFS calls.

Also adds dismissed item tracking — items the player chose to "Leave on
Ground" are remembered so auto-explore won't stop for them again, matching
the existing visited vending machine behavior.

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