Add memory abstraction structures

Introduces three abstractions that build upon the PMP infrastructure
for managing memory protection at different granularities.

Flex pages represent contiguous physical memory regions with
protection attributes, providing arbitrary base addresses and sizes
without alignment constraints. Address spaces organize flex pages
into task memory views and support sharing across multiple tasks.
Memory pools define static regions for boot-time initialization of
kernel memory protection.

Structures are used to enable runtime iteration, simplify debugging,
and maintain consistency with other subsystems. Macro helpers reduce
initialization boilerplate while maintaining type safety.

Author: HeatCrab

include/sys/memprot.h

@@ -0,0 +1,76 @@
+/* Memory Protection Abstractions
+ *
+ * Software abstractions for managing memory protection at different
+ * granularities. These structures build upon hardware protection
+ * mechanisms (such as RISC-V PMP) to provide flexible, architecture-
+ * independent memory isolation.
+ */
+
+#pragma once
+
+#include <types.h>
+
+/* Forward declarations */
+struct fpage;
+struct as;
+
+/* Flex Page
+ *
+ * Contiguous physical memory region with hardware-enforced protection.
+ * Supports arbitrary base addresses and sizes without alignment constraints.
+ */
+typedef struct fpage {
+    struct fpage *as_next;  /* Next in address space list */
+    struct fpage *map_next; /* Next in mapping chain */
+    struct fpage *pmp_next; /* Next in PMP queue */
+    uint32_t base;          /* Physical base address */
+    uint32_t size;          /* Region size */
+    uint32_t rwx;           /* R/W/X permission bits */
+    uint32_t pmp_id;        /* PMP region index */
+    uint32_t flags;         /* Status flags */
+    uint32_t priority;      /* Eviction priority */
+    int used;               /* Usage counter */
+} fpage_t;
+
+/* Address Space
+ *
+ * Collection of flex pages forming a task's memory view. Can be shared
+ * across multiple tasks.
+ */
+typedef struct as {
+    uint32_t as_id;          /* Address space identifier */
+    struct fpage *first;     /* Head of flex page list */
+    struct fpage *pmp_first; /* Head of PMP-loaded list */
+    struct fpage *pmp_stack; /* Stack regions */
+    uint32_t shared;         /* Shared flag */
+} as_t;
+
+/* Memory Pool
+ *
+ * Static memory region descriptor for boot-time PMP initialization.
+ */
+typedef struct {
+    const char *name; /* Pool name */
+    uintptr_t start;  /* Start address */
+    uintptr_t end;    /* End address */
+    uint32_t flags;   /* Access permissions */
+    uint32_t tag;     /* Pool type/priority */
+} mempool_t;
+
+/* Memory Pool Declaration Helpers
+ *
+ * Simplifies memory pool initialization with designated initializers.
+ * DECLARE_MEMPOOL_FROM_SYMBOLS uses token concatenation to construct
+ * linker symbol names automatically.
+ */
+#define DECLARE_MEMPOOL(name_, start_, end_, flags_, tag_) \
+    { \
+        .name = (name_), \
+        .start = (uintptr_t)(start_), \
+        .end = (uintptr_t)(end_), \
+        .flags = (flags_), \
+        .tag = (tag_), \
+    }
+
+#define DECLARE_MEMPOOL_FROM_SYMBOLS(name_, sym_base_, flags_, tag_) \
+    DECLARE_MEMPOOL((name_), &(sym_base_##_start), &(sym_base_##_end), (flags_), (tag_))