feat(fidelity): measure what converting a session into a harness loses

docs/formats/README.md

@@ -57,3 +57,23 @@ than none.
 | [pi.md](pi.md) | pi | `src/harness/pi.rs` |
 | [campfire.md](campfire.md) | Campfire (embeds pi) | `src/harness/campfire.rs` |
 | [simple.md](simple.md) | Simple (txcript's own interchange format) | `src/harness/simple.rs` |
+
+## Conversion fidelity
+
+What converting *into* each harness loses, measured by `txcript::fidelity` on one synthetic session that has every kind of block: signed and unsigned thinking, a Bash call with a description, an edit, an MCP call with a structured result, an error result, a slash command, an image, token usage, stop reasons, per-message models, a title, a git branch. Each cell names the categories that were lost or changed (`ok`: nothing). Items the target only made up, and message timestamps, are not listed.
+
+| Target | Content | Accounting | Metadata |
+| --- | --- | --- | --- |
+| claude_code | ok | changed: stop_reason | ok |
+| codex | lost: tool_args, is_error; changed: tool_name | lost: thinking_signature, usage, stop_reason | lost: title, model |
+| opencode | lost: tool_call, tool_result | lost: thinking_signature; changed: stop_reason | lost: git_branch |
+| pi | lost: tool_call | lost: thinking_signature | lost: title, model, git_branch |
+| campfire | lost: tool_call | lost: thinking_signature | lost: title, model, git_branch |
+| cursor | lost: tool_call | lost: usage, stop_reason | lost: git_branch |
+| cursor_desktop | lost: tool_call, tool_args, tool_result, image | lost: usage, stop_reason | lost: git_branch |
+| grok | lost: tool_call | lost: thinking_signature, usage, stop_reason | ok |
+| fx | lost: tool_call, tool_args | lost: usage, stop_reason | lost: git_branch |
+| antigravity | lost: tool_call, tool_args, tool_result | lost: message_model | lost: model; changed: title |
+| cowork | ok | changed: stop_reason | ok |
+
+The `tool_call` and `tool_result` losses are the slash command, which several formats have no way to record; `tool_name` is it written as `_release`. Hermes, Amp, Simple, and `grok_bot` are measured too (see the golden table in `tests/integration/fidelity.rs`) but are not listed here: `continue` cannot write the first three, and `grok_bot` mints a live agent. The measurement is codec-level: it does not include what a store adds, such as artifact files. It is a regression oracle as well as documentation: `tests/integration/fidelity.rs` fails when the table drifts from the codecs, and prints the rows to paste.

src/dispatch.rs

@@ -0,0 +1,77 @@
+//! Text-level codec dispatch: parse a harness's native session text into the
+//! canonical model, or render the canonical model back to native text, keyed
+//! by a runtime [`HarnessId`].
+//!
+//! No filesystem, no `SQLite`, no subprocess: this is the codec layer alone
+//! (see [`TextCodec`]), so it is available on every target, WASM included. The
+//! WASM bindings and [`fidelity`](crate::fidelity) both route through it.
+
+use crate::harness::{
+    amp, antigravity, campfire, chatgpt, claude_chat, claude_code, codex, cowork, cowork_remote,
+    cursor, cursor_desktop, fx, grok, grok_bot, hermes, opencode, pi, simple,
+};
+use crate::transcript::{Codec, Common, HarnessId, TextCodec, Transcript};
+
+/// Parse `harness`'s native session text into the canonical model.
+///
+/// # Errors
+/// When the text is not that harness's session format.
+pub fn parse_to_common(harness: HarnessId, text: &str) -> crate::Result<Transcript<Common>> {
+    fn go<H: TextCodec + Codec>(text: &str) -> crate::Result<Transcript<Common>> {
+        H::to_common(&H::from_text(text)?)
+    }
+    match harness {
+        HarnessId::ClaudeCode => go::<claude_code::ClaudeCode>(text),
+        HarnessId::ClaudeChat => go::<claude_chat::ClaudeChat>(text),
+        HarnessId::CoworkRemote => go::<cowork_remote::CoworkRemote>(text),
+        HarnessId::ChatGpt => go::<chatgpt::ChatGpt>(text),
+        HarnessId::Codex => go::<codex::Codex>(text),
+        HarnessId::OpenCode => go::<opencode::OpenCode>(text),
+        HarnessId::Pi => go::<pi::Pi>(text),
+        HarnessId::Campfire => go::<campfire::Campfire>(text),
+        HarnessId::Cursor => go::<cursor::Cursor>(text),
+        HarnessId::CursorDesktop => go::<cursor_desktop::CursorDesktop>(text),
+        HarnessId::Grok => go::<grok::Grok>(text),
+        HarnessId::GrokBot => go::<grok_bot::GrokBot>(text),
+        HarnessId::Fx => go::<fx::Fx>(text),
+        HarnessId::Hermes => go::<hermes::Hermes>(text),
+        HarnessId::Amp => go::<amp::Amp>(text),
+        HarnessId::Antigravity => go::<antigravity::Antigravity>(text),
+        HarnessId::Simple => go::<simple::Simple>(text),
+        HarnessId::Cowork => go::<cowork::Cowork>(text),
+    }
+}
+
+/// Render the canonical model as `harness`'s native session text.
+///
+/// # Errors
+/// When the harness cannot represent the transcript (the live read-only
+/// sources, for one).
+pub fn render_from_common(
+    harness: HarnessId,
+    common: &Transcript<Common>,
+) -> crate::Result<String> {
+    fn go<H: TextCodec + Codec>(common: &Transcript<Common>) -> crate::Result<String> {
+        H::to_text(&H::from_common(common)?)
+    }
+    match harness {
+        HarnessId::ClaudeCode => go::<claude_code::ClaudeCode>(common),
+        HarnessId::ClaudeChat => go::<claude_chat::ClaudeChat>(common),
+        HarnessId::CoworkRemote => go::<cowork_remote::CoworkRemote>(common),
+        HarnessId::ChatGpt => go::<chatgpt::ChatGpt>(common),
+        HarnessId::Codex => go::<codex::Codex>(common),
+        HarnessId::OpenCode => go::<opencode::OpenCode>(common),
+        HarnessId::Pi => go::<pi::Pi>(common),
+        HarnessId::Campfire => go::<campfire::Campfire>(common),
+        HarnessId::Cursor => go::<cursor::Cursor>(common),
+        HarnessId::CursorDesktop => go::<cursor_desktop::CursorDesktop>(common),
+        HarnessId::Grok => go::<grok::Grok>(common),
+        HarnessId::GrokBot => go::<grok_bot::GrokBot>(common),
+        HarnessId::Fx => go::<fx::Fx>(common),
+        HarnessId::Hermes => go::<hermes::Hermes>(common),
+        HarnessId::Amp => go::<amp::Amp>(common),
+        HarnessId::Antigravity => go::<antigravity::Antigravity>(common),
+        HarnessId::Simple => go::<simple::Simple>(common),
+        HarnessId::Cowork => go::<cowork::Cowork>(common),
+    }
+}