Fix campaign save isolation bug and add JSON menu loader infrastructure

[Copilot]

Problem

Campaign progress corruption when switching campaigns. Root cause: single shared continue file fx1contn.sav used by all campaigns, causing Campaign A save loads to overwrite Campaign B's continue_level_number.

Changes

Campaign Save Isolation (Critical Fix)

• Per-campaign continue files: fx1_<campaign>_contn.sav instead of shared file
• Campaign name sanitization: Whitelist [a-zA-Z0-9_-] to prevent path traversal
• Helper function: get_safe_campaign_name() for consistent sanitization across save/load

// Before: All campaigns shared fx1contn.sav
// After: Each campaign isolated
fx1_keeporig_contn.sav
fx1_ancntkpr_contn.sav  
fx1_burdnimp_contn.sav

Files: src/game_saves.c (+40 lines)

JSON Menu Loader (Phase 1)

Foundation for data-driven menu system to replace hardcoded GuiButtonInit arrays.

• Parser: JSON → GuiButtonInit arrays using existing centijson library
• Supports: position (with "center" → POS_SCRCTR), size, sprites, text, callbacks
• Memory safe: Proper cleanup, error handling, bounds checking

Files: src/menu_loader.{c,h} (396 lines), Makefile dependency chain

Integration deferred: Frontend wiring, callback registry, string mapping require additional work.

Breaking Change

⚠️ Old fx1contn.sav files won't be used. Players need to replay campaigns or add migration helper on first run.

Security

Path traversal blocked via input sanitization. Campaign names like ../../etc/passwd → etcpasswd.

Original prompt

PROJECT CONTEXT:
You're working on KeeperFX, a C/C++ reverse-engineered game engine (Dungeon Keeper). The codebase uses a custom immediate-mode GUI system (bflib_guibtns) with hardcoded button arrays and binary sprite assets.

BRANCH: feature/ui-campaign-refactor (already pushed to dkfans/keeperfx)

OBJECTIVE:
Implement a data-driven JSON menu system to replace hardcoded UI definitions, and fix campaign save isolation bug where loading saves from different campaigns corrupts progress.

COMPLETED WORK:

Architecture document: docs/ui-system-architecture.md - 15-page spec covering JSON schema, sprite composition (3-piece buttons), 8-frame hover animations, resolution scaling (units_per_pixel), Lua integration, and 8-step migration plan
Reference assets: docs/dev-analysis/assets/ contains 10 game files:
Sprites: frontbit.dat/tab, gui1-64.dat/tab, gui2-64.dat/tab
Backgrounds: front.raw (640×480 paletted), front.pal
Localization: gtext_eng.dat
Save example: fx1g0000.sav
Example JSON: data/menus/campaigns_menu.json demonstrates schema
VSCode extension: Working prototype in separate keeperfx-devtools repo (TypeScript + React, 5 components for visual editing)
YOUR TASKS:

Phase 1: JSON Menu Loader (Priority)

Create src/menu_loader.c + src/menu_loader.h
Integrate cJSON library (already used elsewhere in codebase)
Parse JSON menu definitions into GuiButtonInit arrays at runtime
Support fields: id, position (x/y), size (width/height), sprite, text (localized key), tooltip, callback (Lua function), hotkey
Wire Campaigns menu button to main menu (replace Continue/New/High Scores)
Phase 2: Campaign Save Isolation

Fix bug in game_saves.c lines 220-230: LbFileRead(&game, sizeof(struct Game), gload) loads SGC_GameOrig chunk which overwrites continue_level_number
Create per-campaign save directories: saves/<campaign_name>/ (e.g., saves/keeporig/, saves/ancntkpr/)
Store campaign progress in separate saves//progress.dat files
Remove SGC_GameOrig chunk writes/reads entirely
Update save/load paths in set_continue_level_filename() and related functions
Phase 3: Integration

Use Campaigns menu as proof-of-concept (already has JSON definition)
Test with multiple campaigns: keeporig (original), ancntkpr (Ancient Keeper), burdnimp (Burdened Imp)
Verify save isolation doesn't corrupt progress when switching campaigns
KEY TECHNICAL DETAILS:

Button rendering: 3-piece composition (left cap, tiled center, right cap) in frontend_draw_button() (gui_frontbtns.c lines 723-850)
Animations: 8 frames at 100ms intervals, large_button_sprite_anims[] array
Scaling: scale_ui_value(base) = base * units_per_pixel / 16 (640×480=16, 1920×1080=48)
Sprite loading: get_frontend_sprite(), get_button_sprite() in custom_sprites.c
Campaign definitions: campgns/*.cfg files
BUILD INSTRUCTIONS:

Use WSL with make (see Makefile)
VS Code tasks available: "Default task", "Compile, Copy Files"
Game directory: Read from launch.json cwd field
MINIMAL VIABLE IMPLEMENTATION:
Start with Step 2 of the integration plan (JSON loader only), skip Lua callbacks initially. Get one menu loading from JSON, rendering correctly, before tackling save isolation.

VALIDATION:

Campaigns menu displays with 2+ campaign buttons
Clicking campaign button starts that campaign
Saving in campaign A, switching to campaign B, then back to A preserves both progress values
No SGC_GameOrig chunks in new save files

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