fix(loops): iterate repeater sub-fields and publish bound media URLs

[invisibletype]

Requested by Arlo

Summary

Before: a loop nested inside a loop over a repeater field rendered nothing, so a case study's blocks could not each list their own images. The Loop panel did not offer repeater fields at all. And an Image (or Video) whose source was bound to a media field published the asset id as src (src="zz4G2c…"), with only srcset pointing at real files.

After: repeater items expose their cells as loop item fields, so {currentEntry.layout} reads a block's value and a nested Current entry field loop can iterate that block's images. The Field picker offers repeater fields, and inside a repeater loop it offers that repeater's multi-value sub-fields. Bound images and videos publish the resolved asset URL in src.

How:

• src/core/loops/sources/entryField.ts: projectItem recognises RepeaterItemSchema items and spreads cells into the item fields (the original item stays at value). This is a deliberate shape change: bindings that reached into cells.* now read the sub-field directly.
• LoopPropertiesView.tsx: the entry-field picker resolves context from the nearest enclosing loop. Under an entry.field repeater loop it lists that repeater's sub-fields, otherwise the context tables' collection fields, now including repeaters.
• base.image / base.video: src prefers _resolvedMediaByKey[...].publicPath. Authored URLs resolve to an asset with the same path, so their output is unchanged.
• docs/features/loops.md: picker behaviour plus a "case-study blocks from a repeater" recipe.

Checked end to end on a local SQLite instance: a post type with a blocks repeater (layout select + multi-media images), an entry template with a blocks loop, a data-layout="{currentEntry.layout}" section, and a nested images loop. Every block published only its own images with /uploads/... src values.

Verification

• bun run build: passes
• bun test: the changed suites pass (153 tests). The new repeater and nested-loop tests fail without the fix. The full run on Windows has 24 failures that are unrelated: symlink EPERM, missing server-artifact binaries, one SQLite timeout, and architecture gates on files outside this diff. src/__tests__/architecture gives the same 9 failures with and without this change.
• bun run lint: passes
• Docker/deployment check, if relevant (not relevant)

Checklist

• Tests cover behavior changes.
• Docs were updated when behavior, config, deployment, or public surfaces changed.
• No compatibility shim was added for old pre-release behavior.
• No secrets, local databases, uploads, or generated artifacts are included.

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