fix: pin missing schema versions to their real version (Quantity, EnergyCustomerProfile)

[nirmalnr]

Fixes 81 document URLs (context.jsonld/vocab.jsonld/etc.) across 51 files that referenced Quantity and EnergyCustomerProfile with no version segment at all — e.g. schema.nfh.global/Quantity/context.jsonld instead of schema.nfh.global/Quantity/v2.0/context.jsonld.

• Quantity → pinned to v2.0 (79 occurrences — devkits/demand-flex examples, Postman collections, response fixtures, docs, examples/demand-flex)
• EnergyCustomerProfile → pinned to v1.0 (2 occurrences — inside this repo's own canonical schema definitions: specification/schema/EnergyBillingSummaryCredential/v1.0/context.jsonld and .../EnergyMeterDataGBCredential/v1.0/context.jsonld)

A version-less reference resolves to "whatever the current latest happens to be," which isn't safe to depend on — schema versions here aren't guaranteed backward-compatible (e.g. PriceSpecification has v2.0/v2.1/v2.2 with real field differences, TimePeriod and CategoryCode similarly). A $ref/@context pointing at "latest" can silently start resolving to different, possibly incompatible content the moment a new version ships, with no change on the referencing side — the same failure mode dependency pinning in npm/pip exists to prevent. Pinning to the exact version actually being used makes these references reproducible instead of a moving target.

Separate PR from #487 since this is a distinct problem: missing version entirely, vs. missing the v prefix on an existing version number.

Test plan

• Automated before/after scan: version-less document URLs 81 → 0
• Diff audit: every changed line pins to Quantity/v2.0 or EnergyCustomerProfile/v1.0 — no unrelated content touched
• JSON/YAML syntax validation passes on all 51 changed files

🤖 Generated with Claude Code

[nirmay]

approved.