roadmap: what a Zed submodule pin bump has to check

[simontreanor]

Folds today's second Zed lesson into the ledger entry that already carries the first.

Repointing the submodule at a commit on main satisfied the reviewer and broke the package check:

Error: Incorrect version for extension pyfun (Pyfun)
Expected version: 0.1.0
Actual version: 0.2.0

Moving a submodule pin moves every file inside it, including editors/zed/extension.toml, whose version their package-extensions script validates against the version in their own extensions.toml. Those had agreed in July and drifted since. Fixed by bumping their entry; the check now passes and packages the extension.

The entry now records three things a future pin bump needs:

• squash-merging deletes the commit a pin names, so a pin must be a commit on main
• both version numbers move together
• the Zed extension is versioned on the grammar's schedule rather than the compiler's, so they are never the same number and the pairing has to be checked rather than assumed

It also records that the loud could not find Cargo.toml error in that job's log is benign. It appears identically in the July run that passed, and the real failure is ten lines below it. That cost real time to rule out and would cost it again.