chore: update implementation checklist with lessons learned

[steilerDev]

Summary

Adds recurring patterns surfaced during the reviews in the beta→main range promoted by #2041. Sourced from product-owner/pr-review-patterns.md, product-architect/recurring-patterns.md, and ux-designer/MEMORY.md.

Checklist grows 103 → 121 lines. New entries:

Backend — Data Handling

• Money is stored in major units. The minor-units idiom (amount / 100).toFixed(2) understates every figure by 100× — it reached a bank-facing cover letter in feat(reports): AI-generated usage descriptions and cover letter for bank report wizard #1916. Math.round(x) on major units rounds to whole currency units; the correct "nearest cent" form is Math.round(x * 100) / 100.
• Derived aggregates and their per-item components must be adjusted by the same exclusion rule (feat(reports): AI-generated usage descriptions and cover letter for bank report wizard #1916 emitted parts that summed to more than the stated total).

Frontend — Styling

• :global(.foo) cannot style another module's class: it matches only the literal unhashed string, while className={other.foo} resolves to a hashed name in real builds, so the rule block silently dies. Jest's identity-obj-proxy makes it pass in tests (feat(reports): editable HTML report preview with on-demand PDF export #1909). Use composes:.

Frontend — Shared Components

• A new control in an editor may never reach the export pipeline — grep its state symbol outside the component (fix(reports): improve report PDF UX — paragraph breaks, inline meta, inline notes, column toggles #1959 shipped column toggles that changed only the preview; wiring them to the PDF was the separate story Report column visibility: wire the preview toggles through to the generated PDF #1973 in this very release).
• Preview components must be compared against the exporter's constants, not only design tokens.

Backend — API Conventions

• Reused error codes need feature-neutral copy (feat(reports): AI-generated usage descriptions and cover letter for bank report wizard #1916).
• A wiki edit is not published until pushed — a "documented on the wiki" AC is unmet while the submodule is dirty.

Testing

• Read test titles against their assertions; a title stating the contract over an expectation matching the code is a defect, not coverage (feat(reports): AI-generated usage descriptions and cover letter for bank report wizard #1916).
• Skipped is not passed — a failed upstream job skips its dependents, which is how E2E Gates reported green with all 16 shards skipped on release: promote beta to main #2041 (see E2E Gates reports success when E2E shards are skipped by an upstream failure #2043).
• Confirm an E2E failure is a regression before attributing it to the PR under review.

New section — LLM & Prompt Assembly

• A constraint stated only in the prompt is not a guarantee; tests that pin "the instruction exists" give zero coverage of the outcome (Cover letter overhaul: formatted body, editable signature block, personal sender, professional layout #1932/Enforce the plain-prose guarantee on LLM report content: strip markup in the validator, not only in the prompt #1952).
• Prompt content is usually untested — assert rendered numerics, not just labels (feat(reports): AI-generated usage descriptions and cover letter for bank report wizard #1916).
• Match the coerce-vs-reject policy the field already has (Enforce the plain-prose guarantee on LLM report content: strip markup in the validator, not only in the prompt #1952).
• When hardening is a text transform, the false positives are the risk — domain punctuation collides with markup characters.

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