docs: add CONVENTIONS.md structure & taste guide

[Troublor]

Summary

• Add CONVENTIONS.md at the repo root — a cross-cutting code-organization & taste guide for mega-evm. It covers: a module-by-execution-layer map ("adding X → it belongs in module Y"), file-role conventions (mod.rs as a wiring façade, the factory.rs/result.rs/error.rs/helpers.rs role names), composition idioms (registry-of-trackers, macro-wrapped instruction table, trait factories, spec-gating), when to add a new file vs. extend one, readability/maintainability mechanisms, and a smell list.
• Add a "How to use this doc" split between two review modes — PR gate (don't introduce new violations) and repo-level audit (treat conventions as detection rules and sweep the tree) — plus a repo-wide scan plan turning each convention into a query.
• Add §8 "Known deviations & improvement backlog" — a labeled registry of current tech debt (the StateChangeSource::Transaction(0) placeholder, dead commented-out increment_balances code, duplicated data_size/kv_update tracker bodies, sandbox/ missing its nested AGENTS.md) so repo-level reviews mine documented deviations instead of mistaking them for endorsed convention.
• Wire it in: pointers from AGENTS.md (Core Source Layout) and REVIEW.md (Design and architecture), and add CONVENTIONS.md to the doc-audit.yml / claude.yml agent-file scope so the freshness/correctness audits keep it current.

Test plan

• Docs + CI-prompt-scope only; no code or behavior change (spec:unchanged, api:unchanged).
• markdownlint-cli2 passes with 0 errors; prettier --check 'docs/**/*.md' '*.md' is clean.
• Every code path cited in the doc was verified to exist on main at authoring time.