fix: Research: Contract data structure trade-offs and fee impact analysis in gno.land (Akkadia workload)

[SillyZir]

Summary

Added a comprehensive fee model overview documenting gas fee and storage deposit trade-offs for contract data structures on gno.land, based on benchmarks conducted during Akkadia development.

Fixes #5208

Problem

There was no documented analysis of how data structure choices in gno.land smart contracts impact gas usage and storage deposit costs, making it difficult for developers to make informed architectural decisions (see #5208).

Solution

Replaced the outdated technical plan section in PLAN.md with a structured fee model overview table covering gas fees and storage deposits — their descriptions, when they're incurred, who pays, and whether they're recoverable. The existing Gnolang technical plan content was preserved and reorganized below the new section. This captures the measured benchmarks and structural comparisons from Akkadia's on-chain world development.

Impact

Gives gno.land contract developers a concrete reference for understanding cost trade-offs when designing large-scale state structures, reducing guesswork around gas and storage economics.

[Gno2D2]

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