docs: rewrite HANDOFF for the current tree; drop the three perf/profiling plans

HANDOFF was still describing the 2026-09-08 state (tri-list toggle, relight workaround, pristine
gles11 submodule) -- almost none of which is true now. Rewritten around what the project is, the
environment and rebuild rules that actually bite, the current feature/control set, what diverges in
the two vendored libraries, the three library bugs, the settled findings, the dead ends and the open
items. The perf and profiling plans were superseded status docs; their durable content (tested-unsafe
WC flush, untried levers) is folded in. README's stale relight line fixed.
