feat: add platform fee model with treasury routing on report_revenue

[BigNathan1]

feat: add platform fee model with treasury routing on report_revenue

closes #468

Summary

Adds a configurable, per-offering platform fee so the platform can take a
programmable share of each reported revenue amount, routed to a treasury
address and surfaced transparently in events. Previously report_revenue
allocated the full basis-point budget to holders.

Changes

• New PlatformFeeModel { fee_bps, treasury } contracttype and
  DataKey2::OfferingPlatformFee(OfferingId) storage key.
• set_offering_platform_fee(issuer, namespace, token, fee_bps, treasury):
  admin-only setter. Validates the offering exists and enforces the offering-level
  invariant that fee_bps + aggregate holder share <= 10_000, rejecting
  violations with the new RevoraError::FeeExceedsHolderShare. Emits a
  pfee_set config event.
• get_offering_platform_fee(...) read-only accessor returning the model.
• report_revenue deducts the configured fee on each recorded report and emits
  a plat_fee event carrying (treasury, fee_bps, fee_amount, period_id).

Security and correctness

• Fee and holder allocations share the same 10_000 bps budget; the sum is
  enforced at configuration time so the platform and holders can never claim
  more than 100% of reported revenue.
• The fee is applied only on the paths where a report is actually recorded
  (initial and override). Below-threshold and rejected-duplicate reports return
  early and take no fee.
• fee_bps = 0, zero-revenue reports, and sub-one-unit fees skip the deduction
  and emit no plat_fee event, so indexers can rely on the event being present
  only when a real, non-zero fee was routed.
• Fee math uses checked/saturating arithmetic (amount * fee_bps / 10_000,
  truncating down).
• The new storage key was added to DataKey2 to avoid the primary DataKey
  union's variant cap, and the setter uses a distinct name so it does not
  collide with the existing global platform-fee API.

Tests

New module src/test_platform_fee_model.rs covering: model storage and getter,
config event shape, admin/offering/initialization guards, the
fee+holder-share invariant at and over the 10_000 boundary, full-fee-with-no-
holders, correct fee amount and round-down behavior on report_revenue, the
override path, and the no-event cases (fee_bps=0, no model, zero amount, sub-unit
fee, rejected duplicate, below threshold).

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