chore: scaffold Cargo workspace for the Ghostty rewrite

Start of the full Rust rewrite of Ghostty 1.3.1 (reference source in
../ghostty). Sets up a workspace whose crates mirror Ghostty's Zig modules
(vt, unicode, datastruct, os, input, config, terminal, font, renderer,
termio, core, apprt-gtk) so porting work can proceed in parallel per crate.

Also adds:
- docs/rewrite/{ARCHITECTURE,PLAN,STATUS,AGENT_GUIDE}.md: crate map, core
  design decisions (safe Rust, slab-based PageList, logical capacity
  accounting), phased plan, progress tracker, and porting rules.
- Workspace clippy config relaxing pedantic lints that are pure noise for
  low-level terminal code (casts, long functions).
- Shared vt types needed by several work packages up front: kitty
  keyboard flags (kitty/key.rs) and the CSI separator bitset (SepList).

Test Plan:
- cargo test -p ghostty-vt (3 tests pass)
- cargo clippy -p ghostty-vt --all-targets -- -D warnings

docs/rewrite/AGENT_GUIDE.md

@@ -0,0 +1,56 @@
+# Guide for porting agents
+
+You are porting part of Ghostty 1.3.1 (Zig) to Rust. Read
+`docs/rewrite/ARCHITECTURE.md` first.
+
+- Zig source (read-only reference): `../../../ghostty/src`
+- Rust workspace: `../..`
+
+## Rules
+
+1. **Stay in your lane.** Only edit the crate/modules assigned to you. If you
+   need something from another crate that doesn't exist yet, write a minimal
+   local version, mark it `// TODO(integration): ...`, and mention it in your
+   report. Never rewrite other agents' modules. You MAY add dependencies to your
+   own crate's `Cargo.toml` (prefer well-known crates; use `cargo add -p <crate>`).
+2. **Faithful port.** Preserve behaviour, edge cases, defaults and naming
+   (snake_case of the Zig names). Port doc comments (they are user-visible for
+   config/actions). Don't "improve" semantics. Idiomatic Rust structure is fine
+   (enums for tagged unions, traits for comptime interfaces, `Result` for error
+   unions) as long as behaviour matches.
+3. **Port the tests.** Every Zig `test "name"` in files you port becomes a
+   `#[test] fn name_snake_case()` in a `#[cfg(test)] mod tests` of the
+   corresponding Rust module. Tests that are about Zig-only concerns
+   (allocator failure injection, comptime, C ABI, Metal/macOS) may be skipped —
+   list them in your notes file. Add extra tests when behaviour is subtle.
+4. **No `unsafe`** unless your crate is explicitly allowed (renderer, termio,
+   apprt-gtk), and then only minimal + `// SAFETY:` comments.
+5. **Quality gate before you finish** (must all pass for your crate):
+   - `cargo build -p <crate>`
+   - `cargo test -p <crate>`
+   - `cargo clippy -p <crate> --all-targets -- -D warnings`
+   - `cargo fmt -p <crate>` (rustfmt.toml in repo root; if nightly-only options
+     warn, ignore the warnings)
+   No `todo!()`/`unimplemented!()` left in code paths that are reachable,
+   unless documented in your notes as deferred.
+6. **Do NOT run `git commit`** or any git command that changes state. The
+   orchestrator commits.
+7. Other agents work concurrently in other crates of the same workspace. If the
+   *workspace* build breaks because of another crate, use `-p <crate>` and
+   ignore it. Cargo may block briefly on the build-directory lock; that's normal.
+8. **Write notes** to `docs/rewrite/notes/<crate>-<topic>.md`: what was ported
+   (file → module), skipped tests with reasons, deviations, TODO(integration)
+   items, known gaps.
+9. **Final report** (your last message): ≤ 25 lines — modules done, test counts
+   (ported / skipped), quality-gate status, open issues. The orchestrator reads
+   only this, so be precise and honest about anything incomplete.
+
+## Conventions
+
+- `pub mod` per Zig file; file names snake_case (`PageList.zig` → `page_list.rs`).
+- Error types: `thiserror` is fine; `anyhow` only in binaries/CLI.
+- Logging: the `log` crate (`log::warn!` etc.) where Zig uses `std.log`.
+- Integer types: keep the Zig widths (u16 for cell counts `CellCountInt`, etc.).
+- Zig `packed struct` bitfields → plain structs unless memory layout matters
+  (documented in ARCHITECTURE for `Cell`/`Row`).
+- Prefer `std` collections; `hashbrown`/`smallvec`/`bitflags` OK.