Add Information Architecture split clarification

[Zer0pa-Architect-Prime]

Summary

Adds an ## Information Architecture section between ## What This Is and ## Ship Mechanics that documents the three Zer0paShip program surfaces with explicit ownership scope and non-promotion fences.

Surface	Owns	Does not promote
`Zer0pa/ZeroShip` (this repo)	Public ship-design platform: 15 dimensions, 6 cross-cutting services, 7 public candidate cases, V_01..V_04 audit.	Vessel authority. Class approval. Yard package. Manufacturing release. 23 kn commercial closure.
`Zer0pa/Zero-Class-Vessel-Hull-20098` (public sibling)	Public vessel authority packet for Hull 20098: 14.36 MW @ 23 kn baseline, 10,955 t loaded displacement, 14.35 MW hydro anchor, V_01..V_04 PASS.	Class approval. Yard package. Manufacturing release. Lawful one-unit-out 23 kn commercial closure.
Private working repo (authority-gated)	Implementation, runtime, CAD, raw CFD, OEM/vendor/partner data, premium relaunch authorization.	Anything in the private repo remains gated outside both public surfaces and is not promoted from platform work alone.

Why

Closes the `Repo updates if needed` item from the 2026-05-09 product-page wave dispatch (`§B.1` row for `zeroship`):

Add README IA split clarification if still absent.

The 2026-05-07 landing-card wave-1 §A and 2026-05-09 FPO §A both flag the IA split as documented but not yet rendered in the public README. This PR renders it.

What changed

• `README.md`: added one section (12 new lines, +1,277 bytes). Additive only — no existing content modified or removed.

Truth basis

• Section content cross-checked against `proofs/manifests/CURRENT_PUBLIC_STATUS_PACKET.md` + `PLATFORM-DIMENSIONS.md` + `CROSS-CUTTING-SERVICES.md` (this repo) + Hull-20098 `results/...authority_surface.json` (sibling repo).
• Vocabulary respects Repo Protocol §2: no `BLOCKED` / `FAIL` / `PRIVATE_ONLY` / `INTERNAL` defaults; uses `authority-gated`.
• Closing line echoes the existing `Boundary` row in `Ship Mechanics` table verbatim ("Platform work can enable ship work, but does not promote ship truth by itself.").

Operator checkers

1. Confirm the three-surface scope split reads accurately for the program.
2. Confirm the non-promotion column for the private repo ("premium relaunch authorization") matches your private-side framing.
3. Merge when ready.