Replace the hand-enumerated update/installer/windows-desktop jobs with
a support-matrix generator (scripts/sandbox/generate-e2e-matrix.mjs).
The spec declares every {os, install-method, update-method} combination
a user could be on; generate-matrix expands it and fans out ONE JOB PER
COMBINATION:
* linux combos (curl-bash install x hermes-update/curl-bash rerun)
drive install-e2e-run.yml, still multiplied by the sampled release
tags from pick-releases;
* the windows combo (desktop-installer@latest -> desktop-app) drives
install-e2e-windows-run.yml - the real GUI flow;
* every declared-but-unimplemented combo (all of macOS, the remaining
windows methods) becomes its own visible skipped job, so the
coverage gap is enumerable from the Checks tab and implementing one
is a one-line move into IMPLEMENTED.
Strictness carried into the generator: method ids validate against a
closed set, installer 'versions' arrays only allow 'latest' until a
versioned archive exists, secondUpdate must stay empty until a chained
second-update leg is implemented, and unknown spec keys/routes throw.
Expansion, route filtering, empty-matrix gating, and all seven error
paths verified locally; the dispatch route choice keeps its exact
previous semantics (all/both/update/installer/windows-desktop).
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