Fix crashes and improve input handling

[spacestate1]

Replaces hard crashes (assert/abort) with graceful handling, adds null checks, and maps all Saturn controller buttons.

Files changed

File loading safety

• AzelLib/PDS.cpp getFileSize() (L524-527): Print error message when file cannot be opened (before assert)
• AzelLib/PDS.cpp openFileHandle() (L549-554): Return a valid struct with size 0 instead of crashing when fopen fails
• AzelLib/PDS.cpp loadFile() (L631-634, L673-676): Check for null handle and zero-size files, free resources and return -1 instead of crashing

Dragon model loading

• AzelLib/dragonData.cpp (L544-551): Skip dragon forms with null filename or missing model file instead of crashing

Menu stubs

• AzelLib/mainMenuDebugTasks.cpp (L2767-2770): Only advance menu state if submenu task was created; play error sound if not implemented
• AzelLib/mainMenuDebugTasks.cpp (L2846-2860): Replace assert(0) with Unimplemented() in enemy list, map, system menu, and menu background tasks

Battle system

• AzelLib/battle/battleDebugList.cpp (L62-87): Implement up/down navigation for battle module selection - cycles through valid entries with wrapping and sound effects (was Unimplemented())
• AzelLib/battle/battleManager.cpp (L96): Replace assert(0) with break in unhandled switch default case

Inventory

• AzelLib/menu/inventoryMenu.cpp (L432): Add end iterator for bounds checking
• AzelLib/menu/inventoryMenu.cpp (L449-451): Break out of loop if item array is full (prevents buffer overflow)
• AzelLib/menu/inventoryMenu.cpp (L440): Replace assert(0) with Unimplemented() for berserk category

Other

• AzelLib/town/townCamera.cpp (L94-95): Use default fade color (0x8000) instead of asserting on unknown case
• AzelLib/PDS_Logger.cpp (L5-11): Add cleanupLoggers() to clear all log buffers on shutdown
• AzelLib/PDS_Logger.h (L75): Declare cleanupLoggers()
• PDS/PDS.cpp (L85-87): Call cleanupLoggers() before exit

Input mapping

• AzelLib/renderer.cpp (L448-453): Map gamepad right shoulder, X, left shoulder, back, guide to Saturn C/X/Z/L/R buttons
• AzelLib/renderer.cpp (L505-520): Map keyboard A/S/D/Q/W to Saturn C/X/Z/L/R buttons

Merge note

Safe to merge on its own. All changes are defensive - they prevent crashes and add input buttons. Nothing depends on this PR, and it doesn't depend on any other. Good candidate for merging first.