Implements #76 — let a monorepo root (or any single package) declare its group × version matrix once in a root test/test_groups.toml, run via a new reusable grouped-tests.yml, so CI.yml becomes a thin caller instead of a hand-maintained matrix.

What's here

• scripts/compute_affected_sublibraries.jl — new --root-matrix mode: reads <repo>/test/test_groups.toml (no diff filter, no lib/ required → works for non-monorepos), emits {group, version, runner, timeout, num_threads, continue_on_error}. Adds the continue_on_error per-group field (maps to tests.yml's continue-on-error; for non-fatal groups like ODE's Downstream). Default when absent: a single Core group on [lts,1,pre].
• .github/workflows/grouped-tests.yml — new reusable workflow: detect runs --root-matrix, test fans out to tests.yml@v1 (project: '.') per cell, dispatching the group via group-env-name. Mirrors sublibrary-project-tests.yml's proven passthrough.
• Additive / backward compatible — existing callers and the sublibrary JSON output are unchanged.
• Fixes the Default version matrix -> Core [lts,1,pre], QA [lts,1] #75 regression: the default-groups self-test still expected the old [lts,1.11,1,pre]/[1] matrix, so script-tests went red on master after Default version matrix -> Core [lts,1,pre], QA [lts,1] #75 merged. Updated the expectation + two stale comments. (v1 currently ships this red test — this greens it.)
• Tests: build_root_matrix defaults/fields/continue_on_error, a faithful 46-cell reproduction of OrdinaryDiffEq's embedded root matrix, and a --root-matrix CLI test. Verified locally: all 50 pass; actionlint clean on the new workflow.
• README documents the model, grouped-tests.yml, the continue_on_error field, and the new output mode.

After merge + v1 retag

Migrate each monorepo's root CI.yml to the thin caller + author its root test/test_groups.toml (ODE first as reference; the 46-cell test shows the translation). Non-monorepos can opt in for a near-zero-line CI.yml.

Ignore until reviewed by @ChrisRackauckas.