plan: drop the legacy runtime first (doc 20 D-1)

Carrying the WX_SCHEDULER=0 runtime through the core rewrite would make
every phase dual-path (two code paths per park site, dual-glue builds,
two batteries per gate) for a runtime we intend to delete anyway; git
on the feature branch already provides the rollback the fallback was
for. D-1 now deletes it up front — injector branch, handlesleep.js,
the wxWasmMailboxEnabled gating, the wx legacy twins, and the ablation
builds that pin the old shim — executing doc 17's S5 ledger item 1
early. Interlock and busy gates unaffected (ledger items 2-3, D2/D6).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01TfxKn5utcntBSnxz4ZnYKs

docs/features/async/17-mailbox-scheduler-plan.md

@@ -275,7 +275,8 @@ rollback = the `WX_SCHEDULER=0` build + a per-step tag.
   > legacy → pre-existing local occ_service issue, tracked outside this plan.
   >
   > **Demolition ledger — each deletion with its unlock condition:**
-  > 1. Legacy opt-out + `handlesleep.js` + C++ legacy paths (`startModal`,
+  > 1. **[RESCHEDULED 2026-08-05 → [`20`](20-design-b-core-plan.md) D-1: delete FIRST, on
+  >    the branch, not after CI]** Legacy opt-out + `handlesleep.js` + C++ legacy paths (`startModal`,
   >    `wxWasmRunNestedLoop`, popup pump, `_wxModalResolvers`/`_wxNestedLoopExit`,
   >    `emscripten_async_call` timer entry, timer 17 ms retry branch, `s_wxRunDepth` tick
   >    gate): delete TOGETHER, after CI runs scheduler-only across the full matrix