[P3-E5] Org-internal cap

$1m per 12 months including the flat fee, per organization; automatic sunset (~2 years
after Phase 3, parametrized, no vote — FR-24). Leaves: cap accounting (whose burn counts,
org identity); cap enforcement at purchase; sunset automation; interaction with the
10¢ org-internal tier. Refs: FR-23, FR-24.

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Milestone: Phase 3 (design-level; leaf issues cut when the P3 design settles) · Plan: [phase-3-epics.md](https://github.com/ChainSafe/canton-extending-mainnet/blob/main/docs/planning/phase-3-epics.md)

Start with docs/planning/phase-3-epics.md and the requirements referenced as FR-23 and FR-24. Clarify the P3 design before splitting work into leaves covering cap accounting, purchase enforcement, sunset automation, and interaction with the 10¢ org-internal tier; done means those design decisions are settled and actionable leaf issues exist.

Written by the indexing model from the issue text.