Merge 'Cache reserved_space and page_size values at Pager init to prevent doing redundant IO' from Krishna Vishal

### Problem
Profiling revealed that `usable_space()` calls were consuming 60% of
total execution time for simple SELECT queries, making Limbo
approximately `6x` slower than SQLite for SELECT operations.
The bottleneck was caused by `usable_space()` performing expensive I/O
operations on every call to read `page_size` and `reserved_space` from
the database header, despite `page_size` values being effectively
immutable after database initialization. Only `reserved_space` is
allowed to increase in SQLite.
Evidence: https://share.firefox.dev/44tCUIy
### Solution
Implemented OnceCell-based caching for both page_size and reserved_space
values in the Pager struct:
`page_size: OnceCell<u16>` - Page size is immutable after database
initialization per SQLite specification
`reserved_space: OnceCell<u8>` - Reserved space rarely changes and only
grows, safe to cache
### Performance Impact
Benchmark results: Simple SELECT query time reduced from ~2.89ms to
~1.29ms (~55% improvement)

Closes #1852

core/storage/pager.rs

@@ -9,7 +9,7 @@ use crate::types::CursorResult;
 use crate::{Buffer, LimboError, Result};
 use crate::{Completion, WalFile};
 use parking_lot::RwLock;
-use std::cell::{RefCell, UnsafeCell};
+use std::cell::{OnceCell, RefCell, UnsafeCell};
 use std::collections::HashSet;
 use std::rc::Rc;
 use std::sync::atomic::{AtomicUsize, Ordering};
@@ -222,6 +222,11 @@ pub struct Pager {
     /// Mutex for synchronizing database initialization to prevent race conditions
     init_lock: Arc<Mutex<()>>,
     allocate_page1_state: RefCell<AllocatePage1State>,
+    /// Cache page_size and reserved_space at Pager init and reuse for subsequent
+    /// `usable_space` calls. TODO: Invalidate reserved_space when we add the functionality
+    /// to change it.
+    page_size: OnceCell<u16>,
+    reserved_space: OnceCell<u8>,
 }
 
 #[derive(Debug, Copy, Clone)]
@@ -286,6 +291,8 @@ impl Pager {
             is_empty,
             init_lock,
             allocate_page1_state,
+            page_size: OnceCell::new(),
+            reserved_space: OnceCell::new(),
         })
     }
 
@@ -570,9 +577,15 @@ impl Pager {
     /// The usable size of a page might be an odd number. However, the usable size is not allowed to be less than 480.
     /// In other words, if the page size is 512, then the reserved space size cannot exceed 32.
     pub fn usable_space(&self) -> usize {
-        let page_size = header_accessor::get_page_size(self).unwrap_or_default() as u32;
-        let reserved_space = header_accessor::get_reserved_space(self).unwrap_or_default() as u32;
-        (page_size - reserved_space) as usize
+        let page_size = *self
+            .page_size
+            .get_or_init(|| header_accessor::get_page_size(self).unwrap_or_default());
+
+        let reserved_space = *self
+            .reserved_space
+            .get_or_init(|| header_accessor::get_reserved_space(self).unwrap_or_default());
+
+        (page_size as usize) - (reserved_space as usize)
     }
 
     #[inline(always)]