Export model: unify template-source resolution behind resolve-template-source

[avanelsas]

Summary

Follow-up to #51 and the final piece of the audit's item 6. Three places — `cljs_project`'s `view-context` (template-instance require aliases) + `core-group-body` (root-level template iteration), and `vanilla_js/codegen`'s `template-iteration-expr` — each ran the same three-way `cond` to decide where a template instance's records come from:

1. Explicit `:source-sub` on the instance.
2. Explicit `:source-field`, resolved to the owning group via the field-owner index.
3. Neither set — fall back to the unique stateful host whose `:of-group` collection points at the template.

Three plugins, three branches, one fact about the document. Lift it into `bareforge.export.model/resolve-template-source`, which returns a tagged map: `{:kind :source-sub :sub …}` / `{:kind :source-field :owner-ns … :field …}` / `{:kind :auto-host :owner-ns … :field-name …}`. Each call site now `case`s on `:kind` to format its target-specific sub-ref string.

Behaviour deltas

• `view-context`'s `tpl-child-sub-aliases` and `template-iteration-expr`: pure refactor, identical output.
• `core-group-body` (root-level template entry): previously had no `:auto-host` branch — its three-way `cond` covered `:source-sub`, `:source-field`, and an `:else "::!no-source-for-template"` placeholder. Now it routes `:auto-host` to the same `.subs/` shape the other two sites use. Demo-store doesn't currently trigger this at root, but any future root-level template-instance with no explicit source (and a single `:of-group` host) will resolve correctly instead of falling through to the placeholder.

Test plan

• `clj-kondo --lint src test scripts` — 0 errors, 0 warnings
• `cljfmt check` — clean
• `npx shadow-cljs compile test` — 0 failures, 0 errors (5 new tests pin source-sub precedence, source-field owner resolution, missing-owner nil, demo-store auto-host fallback, and the nil-when-nothing-resolves case)
• `npx shadow-cljs release app` — 0 warnings
• Manual smoke: export demo-store under both plugins; diff codegen output against pre-merge — no diff expected

Item 6 status

With this merged, audit item 6 ("plugins re-derive facts the model should own") is done — both plugins now consume `action-target-of-group-ns`, `resolve-explicit-field-owners`, `find-sub-groups`'s carried source-* fields, and `resolve-template-source` instead of redoing the math per plugin.

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