fix(mv): stop PIXI's BaseTexture from clobbering Bitmap's onload handler

[take-cheeze]

Summary

• The player never actually moved in any MV game ([MV-MOVE] ... moved=false on every probe, on both data/mv-sample and the downloaded data/Lunatic-Core bed) because Scene_Map never became active: the map tileset's Bitmap got permanently stuck at _loadingState: 'requesting'.
• Root cause, found via an instance-level Object.defineProperty watchpoint on the tileset Image's onload (capturing a JS stack on every write): our Image shim (mruby-mvjs/src/mvcanvas.cxx) aliased addEventListener('load'/'error', cb) to the single onload/onerror property. Bitmap#_requestImage attaches its own completion handler that way, but PIXI's BaseTexture also assigns image.onload directly when wrapping an incomplete Image (to learn when it finishes loading) — on a real browser these are independent listener slots that both fire, but our shim let PIXI's later direct assignment silently discard MV's own handler.
• Image now keeps addEventListener-registered listeners and the onload/onerror properties independent, firing both — matching the real DOM contract.
• Also added a generation counter so a stale queued completion from a previous .src assignment can't land on a reused Image instance after a new owner has already re-armed it (MV pools and reuses Image objects via Bitmap._reuseImages).
• Closed the CI assertion gap that let this go unnoticed: the MV movement smokes only checked the process exit code, unlike MZ's moved=true grep. Both MV movement-relevant smoke steps now assert it too.

Test plan

• ./build/mruby/host/bin/mrbtest — 1818/1818 OK, 0 failures
• bash scripts/mz_boot_check.bash — all MZ smokes pass (shared Image shim, unaffected)
• Manual repro against data/mv-sample: [MV-MOVE] ... moved=true consistently across repeated runs
• Manual repro against data/Lunatic-Core (real downloaded game): [MV-MOVE] ... moved=true
• docs/TODO.md M5 write-up updated to reflect the fix (was ⚠️, now ✅); M5 milestone header flipped to ✅
• Changelog fragment added

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