chore(meta): follow the product-group rename into crates.io keywords

[LKSNDRTMLKV]

Two words, and a deadline.

The rename stopped at the manifest

#184 renamed sector to product group across the workspace. The source
followed — the only remaining hits are a doc note recording the rename itself and
the CELEX sector letter, which is a real EUR-Lex term and unrelated.

The published metadata did not:

dpp-plugin-sdk     keywords = ["dpp", "wasm", "plugin", "sdk", "sector"]
dpp-plugin-traits  keywords = ["dpp", "wasm", "plugin", "sector", "compliance"]

Confirmed live on crates.io, not just in the tree.

So the two crates that define what a product group plugin is are indexed
under a word the project abandoned — and sector is one of only five
keyword slots, spent on a term nobody searching for this will type.

🚨 Why this cannot wait for a later cleanup

crates.io keywords are fixed per published version. They cannot be edited
retroactively; they change only when a new version is published. If 0.21.0 ships
as-is, sector is the public vocabulary for another release cycle.

That makes this a pre-release fix rather than a tidy-up — the same shape as
#328, and cheaper.

The change

"sector" → "product-group" in both manifests. Nothing else: no slot added, no
other keyword touched, no description reworded.

Verified against the crates.io constraints — 5 keywords maximum, ≤20 characters
each, [A-Za-z0-9_-] only. Both manifests pass, and cargo metadata parses.

Verification

just check green, 1555/1555.

Noticed, not fixed here

dpp-engines GitHub repository topics contain eu-compl — a truncated
eu-compliance, which dpp-core carries correctly. Nobody browses that topic,
so the tag is inert.

It is a repository setting rather than a file, so it does not belong in a
pull request and I have not changed it. The fix, if you want it:

gh api -X PUT repos/odal-node/dpp-engine/topics \
  -f names[]=circular-economy -f names[]=cryptography \
  -f names[]=digital-product-passport -f names[]=espr \
  -f names[]=eu-compliance -f names[]=gs1-digital-link \
  -f names[]=postgresql -f names[]=rust -f names[]=self-hosted \
  -f names[]=sme -f names[]=sustainability \
  -f names[]=verifiable-credentials -f names[]=wasm

Note that dpp-engine deliberately lacks the open-source topic that
dpp-core has. That is correct — the engine is source-available under BSL,
not open-source — and the command above preserves the omission.