Preserve and validate explicit share_code to prevent registry collisions

[k1412]

Problem

The registry generator falls back to the first four package-name characters when meta.json has no explicit share_code. The current global registry already contains these case-insensitive collisions:

• card: 4 apps
• noir: 3 apps
• void: 2 apps
• zero: 2 apps

The device AppStore resolves a submitted share code by iterating the catalog and accepting the first matching entry, so duplicate codes are ambiguous.

An app cannot reliably prevent the fallback through app-builder.json today:

• czdev publish copies an allow-list of Store fields that omits share_code.
• process-web-submission.yml has the same omission when extracting app-builder.json.
• The documented schema does not list store.share_code.

Concrete reproduction: #116 needs an explicit zsdr in meta.json; the current official generator preview preserves it, but the value had to be inserted outside the normal czdev transformation.

Suggested fix

1. Preserve a validated store.share_code in both czdev publish and web submission metadata extraction.
2. Document the field and a normalized format (for example 4–8 lowercase ASCII letters/digits).
3. Validate uniqueness case-insensitively against the base registry during submission/index generation.
4. Reject duplicates or derive a deterministic unique fallback instead of silently publishing an ambiguous code.
5. Add a registry-generation test covering collisions and explicit codes.

The draft package remains blocked from merge on physical hardware P0 for unrelated release-quality reasons; this issue is specifically about Store metadata integrity.