[UEI EV Charging] Document ON-FULFILLMENT payment type for postpaid metered sessions

[nitinsgawande]

Problem

The current UEI EV charging implementation guide only documents the PRE-ORDER
payment flow, requiring drivers to pay upfront before the session starts. This
creates friction for new users and locks funds in CPO wallets before any energy
is delivered.

Note: ON-FULFILLMENT is already a valid enum value in the layer2 spec
(payment_misc.yaml, energy_EV_1.1.0_openapi_3.1.yaml) — it has simply
never been documented with usage examples in the implementation guide.

Proposed Change

Add implementation guide documentation for ON-FULFILLMENT payment type,
enabling postpaid metered charging:

• Session starts without upfront payment (status: NOT-PAID, amount: "0")
• Real-time cost updates via on_status with a new LIVE-METER tag group
  (energy-delivered, cost-so-far, rate)
• BPP sends UPI collect to driver's VPA after TransactionEvent(Ended)
• Cable remains physically locked until UPI collect is approved
• Final on_update carries SESSION-SUMMARY tag (verified kWh + cost) and
  transitions payment to PAID and fulfillment state to cable-released via
  OCPP UnlockConnector

Use Case

India's DC fast chargers (CCS2 / CHAdeMO) have cable-lock hardware that
prevents drive-offs before payment. The meter-then-pay model is already
familiar to Indian users (auto-rickshaws, Uber). This extends the same
pattern to EV charging without requiring wallet pre-loading or CPO registration.

Enforcement Mechanism

Cable lock (DC chargers): connector is physically locked by the charger until
the BPP sends OCPP UnlockConnector — issued only after receiving UPI
collect confirmation.

I plan to submit a PR with:

• A complete ON-FULFILLMENT postpaid flow covering the full session lifecycle:
  confirm → session start → live metering → session end → UPI collect → cable release
• A new LIVE-METER tag structure for real-time kWh and running cost
  updates pushed to the BAP during an active charging session
• A new SESSION-SUMMARY tag structure carrying verified final energy
  and cost figures sent at session end, before payment is collected
• A cable-released fulfillment state that signals successful UPI
  collection and OCPP connector unlock — completing the postpaid lifecycle
• Taxonomy entries for all new tag codes, fulfillment states, and
  payment params (virtual_payment_address, transaction_id) introduced
  by this flow