Add product price and vector index core

[isomorphisms]

Adds a provider-neutral product/price model to IB, keeps restricted provider offers separate from durable price observations, and adds a rebuildable exact dot-product vector index. The intended vector scalar is 32-bit Float; the currently pinned Idriç compiler exposes Double but not Float, so this executable baseline uses Double only inside rebuildable index state and can switch without migrating canonical records.

For price data specifically, the documented retrieval path prefers ICU + Grease over restricted merchant offer APIs: fetch ordinary web content, extract the minimal price fact with a grep-like pass or a small Idriç parser when structure requires it, and then apply the existing durable-observation vs disposable-current-offer policy. Product metadata, identifiers, and outbound links are not forced through that price path.

[isomorphisms]

Closing this out of the active review queue without deleting its branch or history. The useful impulse and salvage boundary are preserved in the review-residue ledger.

Reason: this bundles unrelated price and vector work; the Double vector sketch is superseded by the tested Float32 filesystem backend in #17, while the price declarations have no acquisition, persistence, fixture, or browser flow yet. A later price slice can reuse the durable-observation versus expiring-offer distinction when it proves one end-to-end path.