Implement heap allocator with management features and unit tests (#22)

This pull request introduces a new boundary-tag free-list heap allocator
for WebCC, replacing the previous bump allocator and improving memory
management across the codebase. The allocator now supports splitting and
coalescing of blocks, in-place growth, and is testable on both WASM and
host builds. Several core containers and formatters are updated to take
advantage of the new allocator's features, and placement new support is
improved for both freestanding and hosted environments.

### Heap Allocator Redesign and Improvements

*
[`include/webcc/core/allocator.h`](diffhunk://#diff-bd10771928744a6c64d378f0bdbc45f48bc1ec2bcdd9cdc900b5854c2b50261cR5-R380):
Replaces the old bump allocator with a boundary-tag free-list allocator,
supporting block splitting on allocation, coalescing on free, and
in-place growth of allocations. Introduces a platform-abstracted backend
for both WASM and host builds, and provides new introspection helpers
for testing.
*
[`examples/webcc_types/example.cc`](diffhunk://#diff-0378a58a312f61c17814815a4b0e3828b493e432b20a87dac94afc7c847a633bL40-R42):
Updates heap visualization code to use the new allocator's backend and
introspection helpers.

### Container and Formatter Enhancements

*
[`include/webcc/core/vector.h`](diffhunk://#diff-7f1d5cfb08de99c4f18ef7467e48b46078082132957e71ec986ca031fc6ccfefR21-R30):
Updates `vector` to use the new `try_grow_inplace` API, allowing
in-place capacity growth without unnecessary reallocations or element
moves.
*
[`include/webcc/core/queue.h`](diffhunk://#diff-c07b9db23a442be44337e1aedad22a371b7f1354813616120140bb98ff81dc61R21-R33):
Updates `queue` to use in-place growth when possible, reducing memory
copying and improving performance in the common case.
*
[`include/webcc/core/format.h`](diffhunk://#diff-9ee51da0bcd04a394f769576442c08ebf423bf9a1e721cdaf571de25250600ceL185-R188):
Refactors `dynamic_formatter` and `hybrid_formatter` to use `realloc`
for buffer growth, leveraging in-place expansion and simplifying buffer
management logic.
[[1]](diffhunk://#diff-9ee51da0bcd04a394f769576442c08ebf423bf9a1e721cdaf571de25250600ceL185-R188)
[[2]](diffhunk://#diff-9ee51da0bcd04a394f769576442c08ebf423bf9a1e721cdaf571de25250600ceR297-R314)

### Placement New Operator Support

*
[`include/webcc/core/new.h`](diffhunk://#diff-82261c7bb3b71b0b04f6d0134769cc4b22047ca257733c55140e76c9186a63d4L2-R3):
Improves placement new/delete support for both freestanding WASM and
hosted builds, conditionally including the appropriate headers or
definitions.
[[1]](diffhunk://#diff-82261c7bb3b71b0b04f6d0134769cc4b22047ca257733c55140e76c9186a63d4L2-R3)
[[2]](diffhunk://#diff-82261c7bb3b71b0b04f6d0134769cc4b22047ca257733c55140e76c9186a63d4R16-R19)
[[3]](diffhunk://#diff-82261c7bb3b71b0b04f6d0134769cc4b22047ca257733c55140e76c9186a63d4R29-R33)

### Test Suite Updates

*
[`tests/run.sh`](diffhunk://#diff-dd0c9cda3917fccebea6cea8d3fb46f46e4fbbeda2d6939864c9aaa9fc9c9571L30-R38):
Adds compilation of new allocator and container tests, and ensures the
correct include paths for the refactored code.

Co-authored-by: io-eric <io-eric@users.noreply.github.com>

Author: io-eric

examples/webcc_types/example.cc

@@ -37,10 +37,10 @@ void log(const char* msg) {
 void draw_heap_viz() {
     if (!canvas_ctx.is_valid()) return;
 
-    uintptr_t base = (uintptr_t)&webcc::__heap_base;
-    uintptr_t current = webcc::heap_ptr;
-    size_t used = current - base;
-    
+    uintptr_t base = webcc::detail::backend_base();
+    uintptr_t current = webcc::detail::heap_ptr;
+    size_t used = webcc::detail::heap_used();
+
     // Get current memory size (in pages)
     size_t current_pages = __builtin_wasm_memory_size(0);
     size_t total_mem = current_pages * 64 * 1024;