docs: reverse gas model concept page

[marc0olo]

Summary

• Explains the reverse gas model: canisters pay, users pay nothing — enabling a Web2-like UX
• Covers what cycles are and why they are pegged to XDR (cost predictability regardless of ICP price)
• Explains ICP → cycles conversion via the CMC and the cycles ledger
• Covers what canisters pay for: compute (best-effort vs. allocated), storage (per byte/second, reservation mechanism), messaging (ingress, inter-canister, creation), special features
• Covers replication factor and how costs scale with subnet size
• Covers developer responsibility: topping up, freezing threshold, deletion risk
• Frames cost predictability as a differentiator vs. gas-auction chains
• Concept page only — no CLI commands; links to cycles-management guide for practical steps

Sync recommendation

Informed by dfinity/portal docs/building-apps/essentials/gas-cost.mdx and docs/building-apps/getting-started/tokens-and-cycles.mdx — hand-written synthesis, concept-only framing per Diataxis.

[Copilot]

Pull request overview

This pull request replaces a TODO placeholder with comprehensive documentation of ICP's reverse gas model. The new content explains how canisters pay for resource consumption using cycles instead of users paying transaction fees, covering key concepts like the XDR peg, cycle conversion via the CMC, cycle costs for compute/storage/messaging, and developer responsibilities.

Changes:

• Replaced placeholder content with detailed explanation of the reverse gas model and its benefits
• Added sections on cycles as a unit of payment, XDR pegging, ICP-to-cycles conversion, and cost breakdowns by operation type
• Included information on developer responsibilities (topping up, freezing thresholds, deletion risks) and cost predictability compared to traditional blockchains
• Added links to related documentation (Cycles Management, Cycles Costs Reference, and Canisters concepts)

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[marc0olo]

Review: Reverse Gas Model

Must fix

• Compute allocation table — 50% row inconsistent with reference: The table uses 1% / 50% / 100% as examples. cycles-costs.md documents 1%, 2%, and 100%. Replace 50% with 2% ("Scheduled every 50 rounds") to align, or add a note clarifying any value 0–100% is valid and these are illustrative.

• Canister creation USD value is stale: Messaging table shows 500B cycles (≈$0.65). At the May 2025 rate (1 XDR ≈ $1.35) used throughout this site, 500B cycles = ~$0.677. Update to ≈$0.68 with a (May 2025) qualifier to match cycles-costs.md.

• Storage USD value is stale: "1 GiB for one year costs approximately 4T cycles (≈$5.35 USD)". The reference shows ~$0.45/month for a 13-node subnet = ~$5.40/year at the May 2025 rate. Update to ≈$5.40 with the same qualifier.

Suggestions

• CycleOps external link: Recommending a specific third-party commercial tool in a concept page is a judgment call. Consider moving it to the cycles-management guide where practical tooling fits more naturally, or framing it as "for example, tools like CycleOps".

Verified

• All internal links resolve: cycles-management.md, cycles-costs.md, canisters.md ✓
• CMC canister ID rkp4c-7iaaa-aaaaa-aaaca-cai and cycles ledger um5iw-rqaaa-aaaaq-qaaba-cai verified against .sources/icskills/skills/cycles-management/SKILL.md ✓
• No dfx references, no .mdx usage ✓
• Frontmatter complete and consistent with body ✓
• Storage reservation threshold (>750 GiB) and subnet capacity (2 TiB) match cycles-costs.md ✓
• Query calls free, 1T = 1 XDR, cycles flow one-way — all accurate ✓
• Content brief fully covered: reverse gas model, XDR peg, CMC conversion, cost categories, predictability ✓

[marc0olo]

Feedback addressed:

• Replaced 50% compute allocation row with 2% ("Scheduled every 50 rounds") to align with cycles-costs.md
• Updated canister creation cost from $0.65 to $0.68 (May 2025 XDR rate, consistent with reference)
• Updated storage cost from $5.35 to $5.40/year (May 2025 XDR rate, consistent with reference)