feat(01-05): commit 2 placeholder north-star assets + IOU (Path C deferral)

User invoked planning-doctrine pushback principle on the 10-20 north-star
curation step. Two 1x1 transparent-PNG placeholders ship with provenance
sidecars marked model_id: 'placeholder' so the validator exercises at >0
assets. Full deferral rationale and resolution path in
.planning/phases/01-foundations-and-doctrine/01-05-IOU.md — to be revisited
at Phase 5 entry (curate then, or amend CONTEXT D-01 if still ceremonial).

assets/north-stars/README.md

@@ -0,0 +1,25 @@
+# North-Star Reference Set
+
+This directory holds the visual contract for AI-generated assets in The Last Garden — the set against which Phase 5+ generations are intended to be measured for style drift (CONTEXT D-01, AEST-08, AEST-09).
+
+## Current state: PLACEHOLDER
+
+The two `placeholder-*.png` files in this directory are **1×1 transparent PNGs**, not real reference images. They exist to exercise the validator pipeline (`scripts/validate-assets.mjs` walking the directory and pairing sidecars correctly) at >0 assets.
+
+Phase 1 deferred the actual curation step. See [`.planning/phases/01-foundations-and-doctrine/01-05-IOU.md`](../../.planning/phases/01-foundations-and-doctrine/01-05-IOU.md) for the decision and resolution path.
+
+## Adding real references
+
+Each asset must have a sibling `<filename>.provenance.json` matching the Zod schema in `scripts/validate-assets.mjs`. Required fields:
+
+- `model_id` — string. Real example: `"stable-diffusion-xl-base-1.0"`. Fallback: `"human"`, `"photograph:cc-by:<photographer>"`, etc.
+- `checkpoint_hash` — string. The exact model checkpoint hash; `"n/a"` if not applicable.
+- `prompt` — string. The full prompt that produced the image.
+- `seed` — string or number.
+- `sampler` — string. e.g. `"DPM++ 2M Karras"`, `"n/a"` for non-AI.
+- `params` — object. Free-form (steps, cfg_scale, dimensions, lora weights, etc.).
+- `provenance_schema_version` — number, optional. Set to `1` for now; Phase 5 may bump.
+
+## Validating
+
+From the repo root: `npm run validate:assets`. CI runs this on every push and PR.