feat(wasm): port Ghostty's wasm32 build

Ports main_wasm.zig as crates/ghostty-wasm (cdylib exporting the Zig
names: atlas_*, face_*, deferred_face_*, shaper_*, config_*), the
web_canvas font face and shaper (backend selected on wasm32), os/wasm
(logging, target, JS interop) and config/Wasm.zig.

C-library dependencies (freetype, harfbuzz, fontconfig, onig, gettext,
D-Bus, rustix) are target-gated. Like Zig's wasm build, link regex
search (StringMap::search_iterator) is unavailable without Oniguruma;
Collection/SharedGrid/harfbuzz are compiled out on wasm32. ghostty-os
switches from forbid to deny unsafe_code for the JS import blocks.

The JS import ABI is currently a documented handle-based interface with
a reference host (js/ghostty_host.js); aligning it with zig-js's exact
import names follows separately.

Test Plan:
- cargo build --target wasm32-unknown-unknown -p ghostty-font \
  -p ghostty-config -p ghostty-terminal -p ghostty-vt -p ghostty-wasm
- cargo test -p ghostty-wasm (node runs the module against a fake canvas)

crates/ghostty-font/src/lib.rs

@@ -8,37 +8,55 @@
 pub mod atlas;
 pub mod backend;
 pub mod codepoint_map;
+#[cfg(not(target_arch = "wasm32"))]
 pub mod codepoint_resolver;
+#[cfg(not(target_arch = "wasm32"))]
 pub mod collection;
 pub mod config_types;
+#[cfg(not(target_arch = "wasm32"))]
 pub mod deferred_face;
+#[cfg(target_arch = "wasm32")]
+#[path = "deferred_face_web_canvas.rs"]
+pub mod deferred_face;
+#[cfg(not(target_arch = "wasm32"))]
 pub mod discovery;
+#[cfg(not(target_arch = "wasm32"))]
 pub mod embedded;
 pub mod face;
+#[cfg(not(target_arch = "wasm32"))]
 pub mod fontconfig;
 pub mod glyph;
+#[cfg(not(target_arch = "wasm32"))]
 pub mod library;
 pub mod metrics;
 pub mod nerd_font_attributes;
 pub mod opentype;
 pub mod shape;
 pub mod shaper;
+#[cfg(not(target_arch = "wasm32"))]
 pub mod shared_grid;
+#[cfg(not(target_arch = "wasm32"))]
 pub mod shared_grid_set;
 pub mod sprite;
 
 pub use atlas::Atlas;
 pub use backend::{Backend, OPTIONS as options};
 pub use codepoint_map::CodepointMap;
+#[cfg(not(target_arch = "wasm32"))]
 pub use codepoint_resolver::CodepointResolver;
+#[cfg(not(target_arch = "wasm32"))]
 pub use collection::Collection;
 pub use deferred_face::DeferredFace;
+#[cfg(not(target_arch = "wasm32"))]
 pub use discovery::{Descriptor, Discover};
 pub use face::Face;
 pub use glyph::Glyph;
+#[cfg(not(target_arch = "wasm32"))]
 pub use library::Library;
 pub use metrics::Metrics;
+#[cfg(not(target_arch = "wasm32"))]
 pub use shared_grid::SharedGrid;
+#[cfg(not(target_arch = "wasm32"))]
 pub use shared_grid_set::SharedGridSet;
 pub use sprite::{Face as SpriteFace, Sprite};
 
@@ -72,11 +90,13 @@ pub enum Presentation {
 }
 
 /// A FontIndex that can be used to use the sprite font directly.
+#[cfg(not(target_arch = "wasm32"))]
 pub const SPRITE_INDEX: collection::Index =
     collection::Index::init_special(collection::Special::Sprite);
 
 /// The default font size adjustment we use when loading fallback fonts.
 ///
 /// TODO: Add user configuration for this instead of hard-coding it.
+#[cfg(not(target_arch = "wasm32"))]
 pub const DEFAULT_FALLBACK_ADJUSTMENT: collection::SizeAdjustment =
     collection::SizeAdjustment::IcWidth;