feat(vt): port the stream layer between parser and handlers

Ports ZIG/src/terminal/stream.zig: the Stream feeds bytes through the
UTF-8 decoder and parser and dispatches every CSI/ESC/OSC/DCS/APC and
C0/C1 control to a Handler as a high-level Action (all 95 Zig
variants). Zig's comptime handler interface becomes a Handler trait with
an associated error type, a required vt() and an optional vt_raw() hook.

Performance: next_slice() processes 4 KiB chunks, finds ESC with memchr
and decodes printable runs in bulk (scalar port of ZIG/src/simd's
index_of / utf8_decode_until_control_seq), plus the CSI parameter fast
path.

Terminals must construct the stream with Stream::new_alloc so the OSC
parser accepts allocating commands (colors, OSC 133), as in Zig.

Minor differences: after a stray UTF-8 byte the rest of the chunk
returns to the fast path, and a spurious Zig warning after CSI 2/5 W is
dropped.

Test Plan:
- cargo test -p ghostty-vt (448 passed; 37/38 stream tests ported,
  the skipped one only exercises the C ABI)

crates/ghostty-vt/Cargo.toml

@@ -7,6 +7,7 @@ license.workspace = true
 [dependencies]
 base64 = "0.23.1"
 log = "0.4.34"
+memchr = "2.8.3"
 thiserror = "2.0.21"