READY: Add lem-tutor interactive tutorial extension

[Catsquotl]

Closes #1967

This PR is ready for review.
What's included:

A working tutorial command that opens an interactive tutorial buffer with progress save/restore across sessions
Lessons 1–4 covering movement, files & configuration, buffers & windows, and editing
Future lessons will be added as separate files
Follows Lem conventions throughout (error handling, lazy-init paths, docstrings)

To load and run:
Since lem-tutor lives inside the Lem source tree, ASDF will find it once the source registry is initialised. In a SLIME REPL inside Lem:
lisp(asdf:initialize-source-registry
'(:source-registry
(:tree "/path/to/lem/")
:inherit-configuration))
(asdf:load-system :lem-tutor)
Then: Alt-x tutorial
Planned follow-up (not blocking merge):

Syntax highlighting for the tutorial buffer
Lesson 5 covering Common Lisp / SLIME
Tests will be added alongside future functionality where applicable

Suggestions on content, structure, or anything else are very welcome — including from the community on Discord.

[vindarel]

👌 I love the progression.

[code-contractor-app]

⚠️ Violations Found (2):

[WARNING] max_total_changed_lines
  Too many lines changed: 711 > 400
  Limit: 400
  Actual: 711

[ERROR] error_handling_rule
  AI check failed: "error_handling_rule"
  ❌ Reason:
    User-facing failure handling reports a message directly instead of using
    `editor-error` as required for messages shown to users.

---

ℹ️ 1 violation(s) dismissed

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[code-contractor-app]

Code Contractor validation failed ❌ — see the sticky comment for full results.

[code-contractor-app]

Code Contractor: error_handling_rule

Contract: contract

AI check failed: "error_handling_rule"

Reason:
User-facing failure handling reports a message directly instead of using editor-error as required for messages shown to users.

---

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[Catsquotl]

I'll see to those soon.

[Catsquotl]

This PR is ready for review.
What's included:

A working tutorial command that opens an interactive tutorial buffer with progress save/restore across sessions
Lessons 1–4 covering movement, files & configuration, buffers & windows, and editing
Future lessons will be added as separate files
Follows Lem conventions throughout (error handling, lazy-init paths, docstrings)

To load and run:
Since lem-tutor lives inside the Lem source tree, ASDF will find it once the source registry is initialised. In a SLIME REPL inside Lem:
lisp(asdf:initialize-source-registry
'(:source-registry
(:tree "/path/to/lem/")
:inherit-configuration))
(asdf:load-system :lem-tutor)
Then: Alt-x tutorial
Planned follow-up (not blocking merge):

Syntax highlighting for the tutorial buffer
Lesson 5 covering Common Lisp / SLIME
Tests will be added alongside future functionality where applicable

Suggestions on content, structure, or anything else are very welcome — including from the community on Discord.

[vindarel]

is there a reason to keep the M-x notation instead of Alt-x? (here and below) For newcomers, we shouldn't be weird, let's show Alt-… as much as possible.

[Catsquotl]

Ah, I somehow thought we changed only Alt-x for commands. And leave the M-letter keybindings. I'll change it. As soon as I can.

Witch was just now.

[vindarel]

we can't use "Alt" in define-keys :S

we'll manage to merge this I have faith.

[Catsquotl]

It's not faith I'm worried about. My own understanding maybe a bit though. You are right, I have this keybindings myself and it does conflict with some modes out there I noticed. M-C-e that is Will think of one that is unused and makes sense, I am if course open to suggestions.