test(session): run the gridwire grid conformance corpus in CI

[kingb]

Makes Ember's agreement with the shared gridwire grid corpus permanent. Until now it rested on a single manual run.

What

• scripts/conformance/fetch-grid-corpus.sh fetches conformance/grid/ from the public gridwire repo at the revision pinned in conformance/GRIDWIRE_REV (currently e977304). Bump the pin on purpose, since new corpus cases are new obligations.
• crates/ember-session/tests/grid_conformance.rs runs Ember's real AlacrittyProjection over every case and compares the decoded deltas.
• CI: a new step in the check job fetches the corpus, runs the test with --ignored, and greps for its pass line, so a renamed or filtered-out test can't pass the step by running nothing.

Why it's shaped this way

This is a conformance test, not a unit test. The corpus isn't Ember's, and what it protects, agreement with other producers of the grid, can't be seen from Ember's own unit coverage. So it:

• fails, never skips, on a missing, empty or unpaired corpus
• fails when the corpus has a field Ember's GridDelta can't represent, instead of silently dropping it on decode
• names the case and the first differing field when it fails, for someone who has never seen this PR

It's #[ignore]d so a plain cargo test doesn't need the network, and it shows up as ignored with a reason that says how to run it.

One real drift fixed

The first run caught a mismatch. gridwire decodes bracketed_paste, mouse_reporting and marks with #[serde(default)]; ember-core's GridDelta didn't, so it couldn't decode a delta that leaves them at their defaults. The attributes now match gridwire's exactly. There's no runtime change, because the local path passes deltas in-process.

Checked that it can fail

Sabotage	Result
Projection stops reshipping the full style table on reset	fails reset-style-carry ([1].new_styles: produced 0 entries, corpus expects 2) and reset-mid-stream
Corpus missing	fails: "grid corpus not found … fails rather than skipping"
A case with no .expect.json	fails, naming the case
An expected delta with a field Ember can't represent	fails: "corpus field .focus_reporting has no counterpart in Ember's GridDelta"

Validation

cargo fmt --check, cargo clippy --all-targets --all-features -D warnings and cargo test --all --all-features are clean. The CI step, simulated locally: all 7 corpus cases agree.

🤖 Generated with Claude Code

https://claude.ai/code/session_014xVjnf8UuQPRQtv2QqgpPK