Unify save system into one save.h/.c; diverge storage format per platform

Renames savefile_t to saveslot_t and folds last session's standalone
settings.h/.c module back in as savemeta_t, so there's one save system
(SAVE.slots[] + SAVE.meta) instead of two parallel ones - while letting
each platform pick its own physical format for the two concepts:

- Linux now writes human-editable JSON (slot0.json, settings.json, ...)
  via yyjson's mutable writer API, so players can hand-fix a bad setting.
- PSP folds meta into the same sceUtilitySavedata binary payload as its
  one save slot (SAVE_SLOT_COUNT_MAX=1 there - a future save picker will
  let players manage multiple named saves via the OS's own browser).
- GameCube consolidates the 3 per-slot memory card files and the separate
  settings file into one combined card file.

Also fixes two bugs surfaced while building this: the CRC finalize step
seeked to a hardcoded offset (only safe for one section per file, breaks
once meta+slots share a buffer), and save.c's async/sync dispatch left an
unconditional fallback call that doesn't exist on PSP-only platforms.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

src/dusklinux/save/savejsonlinux.h

@@ -0,0 +1,147 @@
+/**
+ * Copyright (c) 2026 Dominic Masters
+ *
+ * This software is released under the MIT License.
+ * https://opensource.org/licenses/MIT
+ */
+
+#pragma once
+#include "error/error.h"
+#include "yyjson.h"
+
+/**
+ * Small helper around a yyjson mutable document, used to build up a save
+ * file's fields one at a time before writing it out. Schema-agnostic -
+ * knows nothing about saveslot_t/savemeta_t; the caller supplies field
+ * names and values.
+ */
+typedef struct {
+  yyjson_mut_doc *doc;
+  yyjson_mut_val *root;
+} savejsonwriterlinux_t;
+
+/**
+ * Creates a new mutable JSON document with an empty root object.
+ *
+ * @param writer Writer to initialize.
+ * @return An error if the document can't be allocated.
+ */
+errorret_t saveJsonWriterInitLinux(savejsonwriterlinux_t *writer);
+
+/**
+ * Adds a "version": <uint> field to the root object.
+ */
+void saveJsonWriterAddUInt32Linux(
+  savejsonwriterlinux_t *writer, const char_t *key, const uint32_t value
+);
+
+/**
+ * Adds a float field to the root object (written as a JSON number).
+ */
+void saveJsonWriterAddFloatLinux(
+  savejsonwriterlinux_t *writer, const char_t *key, const float_t value
+);
+
+/**
+ * Adds a string field to the root object. The value is copied into the
+ * document, so the caller's buffer doesn't need to outlive the call.
+ */
+void saveJsonWriterAddStringLinux(
+  savejsonwriterlinux_t *writer, const char_t *key, const char_t *value
+);
+
+/**
+ * Adds a JSON array of booleans as a field on the root object.
+ */
+void saveJsonWriterAddBoolArrayLinux(
+  savejsonwriterlinux_t *writer, const char_t *key, const bool_t *values,
+  const size_t count
+);
+
+/**
+ * Adds a JSON array of unsigned 8-bit integers as a field on the root
+ * object.
+ */
+void saveJsonWriterAddUInt8ArrayLinux(
+  savejsonwriterlinux_t *writer, const char_t *key, const uint8_t *values,
+  const size_t count
+);
+
+/**
+ * Pretty-prints the document to the given file path, creating or
+ * truncating it.
+ *
+ * @param writer Writer holding the document to write.
+ * @param path Destination file path.
+ * @return An error if the write fails.
+ */
+errorret_t saveJsonWriterSaveLinux(
+  savejsonwriterlinux_t *writer, const char_t *path
+);
+
+/**
+ * Frees the document. Safe to call even if saveJsonWriterInitLinux()
+ * failed partway.
+ *
+ * @param writer Writer to dispose.
+ */
+void saveJsonWriterDisposeLinux(savejsonwriterlinux_t *writer);
+
+/**
+ * Reads and parses a JSON file, returning its root object.
+ *
+ * @param path File path to read.
+ * @param outDoc Receives the parsed document (must be freed via
+ *   yyjson_doc_free() once done, regardless of found/error outcome).
+ * @param outRoot Receives the root object, or NULL if not found.
+ * @param found Set to true if the file exists, false if it does not
+ *   (not finding the file is not an error).
+ * @return An error if the file exists but fails to parse.
+ */
+errorret_t saveJsonReaderOpenLinux(
+  const char_t *path, yyjson_doc **outDoc, yyjson_val **outRoot,
+  bool_t *found
+);
+
+/**
+ * Reads a uint32 field, falling back to defaultValue if the key is
+ * missing or not a number - a hand-edited file shouldn't hard-fail the
+ * whole load over one bad/missing field.
+ */
+uint32_t saveJsonReadUInt32Linux(
+  yyjson_val *root, const char_t *key, const uint32_t defaultValue
+);
+
+/**
+ * Reads a float field, falling back to defaultValue if the key is
+ * missing or not a number.
+ */
+float_t saveJsonReadFloatLinux(
+  yyjson_val *root, const char_t *key, const float_t defaultValue
+);
+
+/**
+ * Reads a string field into out, falling back to defaultValue if the key
+ * is missing or not a string. Always null-terminates.
+ */
+void saveJsonReadStringLinux(
+  yyjson_val *root, const char_t *key, char_t *out, const size_t maxLen,
+  const char_t *defaultValue
+);
+
+/**
+ * Reads a JSON array of booleans into out, up to count entries. Missing
+ * key, non-array value, or a shorter array all leave the remaining/all
+ * entries untouched (caller should zero the buffer first).
+ */
+void saveJsonReadBoolArrayLinux(
+  yyjson_val *root, const char_t *key, bool_t *out, const size_t count
+);
+
+/**
+ * Reads a JSON array of unsigned 8-bit integers into out, up to count
+ * entries. Same forgiving semantics as saveJsonReadBoolArrayLinux().
+ */
+void saveJsonReadUInt8ArrayLinux(
+  yyjson_val *root, const char_t *key, uint8_t *out, const size_t count
+);