docs: add the GR compute-launch frontier page and align execution status across the tree

New page docs/04-implementation/03-execution-frontier.md describes the chain
that must fire for a guest CUDA launch (guest push -> GPFIFO/front-end ->
inline launch-descriptor install -> engine read/dispatch -> completion),
where the leading edge is (the descriptor install path), how we know
(front-end vs engine-authored observables and their limits), what is
exonerated (context validity at dispatch, destination window mapping,
fetch alignment, shared-buffer rule, parenting), the ranked issues (verbatim
prelude targets, missing descriptor-virtualization control, sentinel
hygiene), live investigation lines and labelled theories.

Align every page that still said GR execution is blocked on engine-context
validity to the current frontier, and add the new page to the README reading
order (row 15, later rows renumbered). Docs only; no code.

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