#205 docs: establish Effect-native replacement foundation

Purpose

Docs-only planning foundation for #174 and specification #180. Records the accepted Effect-native replacement contracts, retained research and migration/acceptance map, and retires superseded prerelease guides. This is not one of implementation tickets #181–#204.

Stack

• Base: main at c343410fc855af6cc2fb472ff2f2d2ba466becf6
• Head: docs/effect-native-planning-checkpoint at 9e1917971f09252ef0f6629720cbbc882c6e87d9
• Commits: 8313b31, 0064d67 and 9e19179
• Next stack PR: #206 (#181 native baseline), followed by #207 (partial #170 contracts). #181 is stack-satisfied at its reviewed/tested ancestor for descendant work; all PRs remain draft and unmerged.

Verification and limitations

• Coordinator reran git diff --check c343410fc855af6cc2fb472ff2f2d2ba466becf6 0064d6745f276d8e2e895d0ffdc282d13e70d562: exit 0.
• Coordinator reran installed oxfmt --check on all 52 added/modified files: exit 0.
• CI verify passed: lint/format/types/build (28 tasks, 2 cached), test pipeline (17 tasks, 9 cached), docs prerender (6 pages). CI used merge ref cd9e23c246f4cabb5d278371e24f0b25320b90c4; coordinator verified its tree equals the then-head 0064d67. This verifies the existing implementation with the docs delta, not the native replacement.
• Follow-up 9e19179 replaces eight duplicated policy bullets with an ADR index (-5 lines). Parent format/whitespace checks pass; all index anchors resolve. Detailed protocols and historical evidence are unchanged. The prior CI result above is bound to 0064d67. Current CI verify also passed for 9e19179; parent verified merge ref 8699c96f361bceff0aa160a0437961d04a07a7f2 has the same tree.
• Installed compiler verified as 7.0.2+effect-tsgo.0.36.5; repository Effect remains 4.0.0-rc.108. No dependency change.
• Independent Standards/Spec reviews found no issues on 0064d67. The separate Ponytail review found duplicated policy text; that finding is resolved on 9e19179, with independent follow-up confirming all removed policies remain in ADR-0056 and no new normative policy was introduced. Parent carries earlier coverage forward only for unchanged content; the new index has its own checks/review.
• Still draft and awaiting external review; silence is not approval. The new-head bounded feedback window ended with CI green and no external reviews or threads. Changesets bot reminder is informational for this docs-only PR. No production replacement, durable backend, native-platform or machine-failure gate is claimed complete.
• #174/#180 remain open. No merge/release is requested or performed.