cleanup: drop zimmerframe devDep and the seen-set idempotency guard

[NullVoxPopuli-ai-agent]

Summary

zimmerframe is no longer used since the walker switched to visitorKeys in #49. This cleans up the remaining traces and removes the seen WeakSet, which guarded against a double-fire case that the new walker can't reach.

Changes

• Drop zimmerframe from devDependencies. Not in dependencies since perf: drop zimmerframe from outer-AST walk, always use visitorKeys #49.
• Rewrite tests/glimmer-comments.test.js to use toTree's visitors API instead of importing walk from zimmerframe. Same assertions, same coverage.
• Update stale comment in tests/custom-parser-ts.test.js (was referencing zimmerframe's cloning behavior).
• Remove the seen WeakSet from walkWithKeys. The guard existed because zimmerframe's for (const key in node) could follow enumerable parent back-links and revisit a node. The visitorKeys walker only follows declared child slots, so this class of double-fire is no longer reachable. Visitors that relocate nodes into non-child slots (e.g. program.comments) still never revisit because those slots aren't in visitorKeys either.

Perf impact

Tiny. The seen set was costing ~0.3% of parse time in ember-eslint-parser's bench (WeakSet.add on every outer-AST node). Dropping it trims that.

Test plan

• All 189 existing tests pass
• The relocation test (moves a Glimmer comment into program.comments and removes it from the template) still passes — it moves into program.comments, which isn't in visitorKeys, so the walker never revisits.

🤖 Generated with Claude Code

[NullVoxPopuli]

this is still useful for testing