feat(sdk): promote SPEC-017 from Warning to Error at spec 2.0.0

[GarthDB]

Context

Tracked follow-up from issue #941 (property field migration path) and the migration policy landed in spec/token-format.md.

Work

At the spec 2.0.0 cut, flip SPEC-017 (string-name-tech-debt) from warning to error:

• sdk/core/src/validate/rules/spec017.rs:40 — change Severity::Warning → Severity::Error.
• Update rule catalog in packages/design-data-spec/rules/rules.yaml if the severity is declared there.
• Add a changelog entry noting this is a breaking validator change.
• Verify design-data validate packages/tokens/src --names-dir packages/token-names/names/ emits the expected error count (or zero if all string-name tokens are remediated before the 2.0.0 cut).

Preconditions

• All string-named tokens in packages/tokens/naming-exceptions.json must be converted to structured name objects in @adobe/token-names (sidecar), or the dataset migration must be complete, before this PR ships.
• The validator must be invoked with --names-dir packages/token-names/names/ so SPEC-017 can see the name objects (they are no longer inline in packages/tokens/src/ as of feat(taxonomy): extract token name objects to @adobe/token-names sidecar package #972).
• This PR must target the 2.0.0 release branch or be merged with a major-version bump.

References

• Issue RFC #806: property field migration path #941 (migration policy)
• PR feat(taxonomy): extract token name objects to @adobe/token-names sidecar package #972 (sidecar extraction — name objects moved to @adobe/token-names)
• docs/proposals/011-string-name-escape-hatch.md (SPEC-017 origin)
• packages/design-data-spec/spec/evolution.md (severity tightening = major change)