Move global item collected-state, deadzone, and story flags into save file

Adds three new pieces of save-file state, all following the same shape:
the save file is the single source of truth, not a separate live runtime
copy that gets synced in/out.

- globalitemstore.h/.c: per-global-entity-ID "collected" flags
  (savefile_t.globalItemCollected), so a global item entity's init
  callback can check whether it was already picked up in a prior session
  without needing to keep the entity itself alive to remember that.

- Gamepad deadzone: removed input_t.deadzone entirely. The setting UI and
  every platform's actual deadzone-applying code (inputGetDeadzoneDolphin/
  SDL2, previously hardcoded per-platform literals that the settings menu
  didn't actually affect) now read savefile_t.deadzone directly via
  saveGet(SAVE_ACTIVE_SLOT). Default lives in savefile.h
  (SAVE_DEADZONE_DEFAULT), stamped onto every slot in saveInit().

- Story flags: STORY_FLAG_VALUES (a live, codegen-initialized array) is
  replaced by savefile_t.storyFlags, read/written via the existing
  storyFlagGet()/storyFlagSet() call sites (now macros/functions over the
  active save file instead of a separate array). tools/story.py now
  generates STORY_FLAG_DEFAULTS (const) instead; storyFlagInitDefaults()
  stamps those onto a save the first time it's used (file->exists false),
  called from rpgInit().

Added SAVE_ACTIVE_SLOT (0) to savefile.h as the one shared "which slot is
actually being played" constant, replacing three different local/implicit
0s (uigamemenu.c, rpg.c, and now the settings/input call sites).

Verified round-trip on Linux (all three together in one save/load cycle);
both Linux and PSP build clean.

tools/story.py

@@ -38,7 +38,11 @@ out += [
   "  STORY_FLAG_COUNT",
   "} storyflag_t;",
   "",
-  "static storyflagvalue_t STORY_FLAG_VALUES[STORY_FLAG_COUNT] = {",
+  "// Stamped onto a save file's storyFlags the first time it's used (see",
+  "// storyFlagInitDefaults()) - not a live value array. Live flag state",
+  "// lives entirely in the save file (savefile_t.storyFlags), read/written",
+  "// via storyFlagGet()/storyFlagSet() - see storyflag.h.",
+  "static const storyflagvalue_t STORY_FLAG_DEFAULTS[STORY_FLAG_COUNT] = {",
 ]
 for flag in flags:
   out.append(f"  [{flag_enum(flag['id'])}] = {flag['initial']},")