docs: README usage/install section; ci: test full Python version matrix

[dmccoystephenson]

Summary

• Add an Installation section (pip install graphik) and a Usage section documenting all public Graphik methods and color constants to README.md — it doubles as the PyPI long description and previously had no install/usage info.
• Fix the README Projects link, which pointed at the pre-transfer Preponderous-Software/Graphik wiki path, to the current Stephenson-Software/graphik location.
• Replace the single pinned Python 3.13 CI job with a matrix (3.9-3.13), covering every version pygame ships a prebuilt wheel for, so the declared requires-python range is actually exercised.
• Bump requires-python from >=3.8 to >=3.9 in pyproject.toml since 3.8 is EOL (Oct 2024) and isn't tested -- keeps the declared floor and the tested floor in agreement per issue ci: test workflow verifies only Python 3.13 while pyproject claims >=3.8 support #20's acceptance criteria.

Test plan

• python3 -m py_compile src/main/python/preponderous/graphik/graphik.py
• python3 -c "import sys; sys.path.insert(0,'src/main/python'); import preponderous.graphik as g; print(g.__version__)" -> 0.2.0
• python3 -m pytest -q -> 19 passed (conftest.py already forces SDL_VIDEODRIVER/SDL_AUDIODRIVER=dummy, so this runs headless without extra env vars)
• CI matrix itself (3.9-3.12 legs) can only be verified once this runs on GitHub Actions -- the sandbox interpreter here is 3.8, so those legs aren't locally reproducible; the 3.13 leg mirrors the previously-passing pinned job.

Note: also closed #5 (packaging/distribution) separately during this cycle -- it was fully resolved by prior merged PRs (pyproject.toml, single version source, PyPI publish workflow) with only out-of-scope consumer-migration follow-up remaining.

Closes #19
Closes #20

[dmccoystephenson]

Self-review rubric:

• Scope: PASS — README.md and pyproject.toml changes each map to docs: README has no installation or usage section and links to the pre-transfer repo path #19/ci: test workflow verifies only Python 3.13 while pyproject claims >=3.8 support #20; pyproject.toml's requires-python bump is required by ci: test workflow verifies only Python 3.13 while pyproject claims >=3.8 support #20's own acceptance criteria ("requires-python and the versions actually tested agree"), not drive-by.
• Tests-new: N/A — no new public method added, docs/CI-only change.
• Tests-fix: N/A — not a bug fix; nothing to stash-and-run.
• Sibling structure: N/A — no new files.
• Sibling renames: N/A — no identifiers renamed.
• Docs: PASS — README now documents install, usage, and the full public method/constant list against the actual graphik.py source (cross-checked method names/signatures against the file); Projects link points at Stephenson-Software/graphik.
• Issue resolution: PASS — docs: README has no installation or usage section and links to the pre-transfer repo path #19's three acceptance criteria (install instructions, method list + example, no stale link) and ci: test workflow verifies only Python 3.13 while pyproject claims >=3.8 support #20's three acceptance criteria (matrix beyond one interpreter, requires-python matches tested versions, no from-source pygame build) are each met.
• manual validation: PASS — py_compile, import smoke test (preponderous.graphik.__version__ == 0.2.0), and pytest -q (19 passed) all green on the PR head.
• camelCase: N/A — no new/renamed methods.
• Backward-compat: PASS — no existing public member touched.
• Headless: PASS — no new tests; existing suite still runs via conftest's dummy SDL drivers.
• Version sync: PASS — _version.py (0.2.0) untouched; the requires-python bump is a support-range change, not a release version change.
• No new deps: PASS — no imports added.

Note: this PR touches .github/workflows/test.yml, which is on this loop's do-not-auto-merge list (CI config changes affect downstream review/automation gates) — leaving open for human review rather than auto-merging, per the skill's Phase 8 gate.