Fix default templates crashing when card model is transiently undefined

[christse]

Problem

Production host intermittently crashes while rendering a card:

TypeError: Cannot read properties of undefined (reading 'constructor')
    at get cardInfoFieldDisplayNames (https://cardstack.com/base/default-templates/isolated-and-edit:10:50)

The default isolated/edit template derives the card class from this.args.model.constructor inside tracked getters, and dereferences this.args.fields unguarded. The host can render a format component for a tick while the model instance is still resolving — on initial load, and on a store re-resolve when an incremental index invalidation lands under an open card — so the first tracked getter throws and takes down the render. Reproduced repeatedly on a realm with heavy linksTo graphs and frequent CLI pushes (live invalidations of open cards).

Fix

• Take the class from @cardOrField instead of model.constructor. Every format component already receives the class as an arg, independent of the instance — head.gts and embedded.gts use it for exactly this reason. isolated-and-edit.gts and field-edit.gts were the two templates digging it out of the instance instead.
• Guard the @fields dereferences (displayFields, isThemeCard) and render the CardInfo edit header and notes footer only when @fields.cardInfo exists — invoking <@fields.cardInfo.notes /> with undefined @fields throws "attempted to invoke undefined component".

Behavior is unchanged whenever model/fields are present; a render pass with no data now produces an empty shell for that frame instead of throwing. This matches the defensive convention card-info.gts already follows throughout.

Verification

• ember-template-lint clean on both files
• host lint:types error count identical before/after (7 pre-existing, unrelated .at() lib-target errors)
• The pre-commit eslint parse errors on these files are pre-existing for all packages/base gts files (the eslint config doesn't parse <template> there; the package's own lint is ember-template-lint, which passes)
• Existing tests asserting the notes footer (card-basics-test) are unaffected by construction — the guards only change behavior when fields are absent, which previously crashed

Tracked in CS-12330.

🤖 Generated with Claude Code

[christse]

[Claude Code 🤖] Closing: this fix was made while working on a userland realm project, and platform changes shouldn't ride along with that work. The affected realm is shielded by a realm-side guarded default template, so nothing depends on this landing. CS-12330 remains the tracking ticket for the underlying host issue (default templates crash when the card model is transiently undefined); the fix-default-template-undefined-model branch stays available if the platform team wants to pick the @cardOrField approach up.