docs(cases/mariadb): add semisync persist alpha.83 to alpha.88 evolution case

[weicao]

Summary

Adds a MariaDB-specific case study under docs/cases/mariadb/ describing how the semisync persistence fix took 4 alpha versions (alpha.83 -> .84 -> .85 -> .86/.87 -> .88) to close cleanly. Generic methodology stays in troubleshoot/addon-reconfigure-set-global-persist-race-guide.md; this PR carries only the case history.

Coverage

• Timeline of what each alpha tried and why it was insufficient
• Final boundary: the 4-layer defense that closed the issue
• Six cross-team takeaways readers can apply to non-MariaDB addons
• Artifact sha anchors per stage so readers can verify the evidence

Timeline

1. alpha.83 — false-success surfaced (OpsRequest Succeed but pod1 SQL OFF)
2. alpha.84 — !includedir in chart-rendered my.cnf rejected by KB INI parser
3. alpha.85 — /var/lib/mysql/conf.d/ not auto-scanned by mariadbd
4. alpha.86/.87 — parse smoke against mariadbd failed because kbagent runtime PATH lacks server binary; set -e + var=$(failing_cmd) silently exited
5. alpha.88 — drop parse smoke, keep --defaults-extra-file + loader !includedir + per-param atomic writes; closes cleanly

Cross-references

• troubleshoot/addon-reconfigure-set-global-persist-race-guide.md — engine-neutral methodology
• cases/mariadb/mariadb-reconfigure-set-global-without-persist-race-case.md — single-point false-success case for the same root cause
• troubleshoot/addon-debug-misjudgment-checklist-guide.md item [style-harmonize] add minimal README for repo orientation #2 — set -e + var=$() silent exit
• troubleshoot/addon-syncer-image-version-pin-guide.md — alpha.85 bundled the image.syncer.tag pin as a side-effect

Index updates

• docs/SKILL-INDEX.md MariaDB case count bumped 16 -> 17

Test plan

• Markdown renders without dead links
• Case is engine-specific and does not duplicate the engine-neutral methodology guide
• 116 lines, under the 120-line case budget
• Standard metadata block present
• Public hygiene grep clean
• Rebased on current main