Built heap allocation

docs/heap.md

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+# The kernel heap
+
+The [frame allocator](frame-allocator.md) hands out fixed 4 KiB physical frames;
+the [VMM](paging.md) maps pages into virtual addresses. The **kernel heap** sits on
+top of both to provide what the rest of the kernel actually wants: `alloc(n)` /
+`free(p)` for arbitrary byte sizes. It's the first real consumer of `map()`, and
+the thing that unlocks dynamic data structures — lists, hash maps, driver state,
+eventually a process table.
+
+It's generic kernel code (`src/heap.zig`): the allocator logic is
+architecture-neutral, using `arch.mapPage` and the frame allocator underneath.
+
+## A growable free-list allocator
+
+The algorithm is a classic **first-fit free list**:
+
+- The heap owns a virtual region. Free space is tracked as an **address-ordered
+  singly linked list** of free blocks; each block begins with a 16-byte header
+  (`size`, and a `next` link used while free).
+- **alloc(n)** walks the list for the first block big enough. If the block is much
+  larger it's **split** — the front becomes the allocation, the remainder stays
+  free. If nothing fits, the heap **grows** (below) and the search retries.
+- **free(p)** finds the block header just before `p` and inserts it back into the
+  list, **coalescing** with the physically adjacent free blocks on either side so
+  the space can be reused as one region rather than fragmenting away.
+
+Allocations are 16-byte aligned; larger alignments aren't supported yet (the
+`std.mem.Allocator` `alloc` returns `null` for them).
+
+## Growing on demand
+
+The heap lives in the **higher half** of the address space (virtual base
+`0xFFFF_8000_0000_0000`) — unmapped, well clear of the identity-mapped low half,
+and leaving the low half free for a future user address space. (That base is
+x86_64-canonical; another architecture would pick its own.)
+
+When the free list can't satisfy a request, `grow` extends the mapped region: it
+pulls fresh frames from the [frame allocator](frame-allocator.md) and `map`s each
+onto the end of the heap, then adds the new span as a free block (coalescing with
+the current tail). So the heap starts at one page and expands page-by-page as
+demand requires, up to a cap. This is exactly what the VMM's on-demand `map` was
+built for.
+
+## A std.mem.Allocator
+
+The heap is exposed as a **`std.mem.Allocator`** (`heap.allocator()`), Zig's
+standard allocator interface. That's a deliberate multiplier: it means the whole of
+Zig's standard library — `ArrayList`, `AutoHashMap`, `std.fmt.allocPrint`, and the
+rest — works directly on the kernel heap, no bespoke containers required.
+
+## Verifying it
+
+The `heap` test (see [testing.md](testing.md)) exercises the allocator end to end:
+
+```
+[PASS] alloc 4096 bytes
+[PASS] heap memory is writable and reads back
+[PASS] freed block is reused              <- free list + coalescing works
+[PASS] many allocations (heap growth) stay valid   <- grow() maps fresh frames
+[PASS] std.ArrayList on the kernel heap    <- std containers work on it
+```
+
+The "freed block is reused" check (free then re-alloc returns the same address) is
+the proof that free and the free list actually work, not just alloc; "heap growth"
+forces allocation past the initial page so `grow`/`map` runs; and the `ArrayList`
+check is the std-integration payoff.
+
+## What's next (not done here)
+
+- **Thread/interrupt safety.** The heap assumes a single caller — no lock yet.
+  It's safe now (nothing allocates from interrupt handlers), but threads or an
+  allocating IRQ handler will need a lock (or `cli` around the critical section).
+- **Larger alignments** than 16 (for page-aligned buffers, DMA regions).
+- **`resize`/`remap` in place**, so growing an `ArrayList` needn't always copy.
+- **Reclaiming empty tail pages** back to the frame allocator when the heap shrinks.