[gmatossian]

The equality and hash-code topic could make readers apply the key/set-element mutation rule to ordinary map values. This change puts the three positions side by side and shows that replacing a value under a stable key needs no remove-first step.

What changed

• Add a compact comparison of HashSet elements, HashMap keys, and HashMap values.
• Highlight the safe mutation rule for hash-based keys and set elements.
• Add a short Java value-replacement example and authoritative Java 21 references.
• Preserve the existing equality/hash contract and sorted-collection guidance.

Verification

• npm run check — passed (176 unit/tool tests and 38 browser tests).
• git diff --check — passed.
• Oracle Map and Set reference links — HTTP 200.
• Rendered topic reviewed at desktop and 390 px narrow width.

Review

• Deliberate scoped final-diff review completed before push; no blocking findings.
• Independent review omitted as disproportionate for this bounded documentation-only change.
• Codex drafted the content change and performed the delegated verification and GitHub workflow; Gabriel reviewed and accepted the rendered direction.

Closes #117