docs: clarify graphik is not yet published to PyPI

[dmccoystephenson]

Summary

• The README's Installation section stated pip install graphik as if it already worked, but the package has never been published to PyPI (pip download graphik finds no matching distribution — confirmed against the live PyPI index).
• Clarifies the current state and gives an interim pip install git+https://... install path, while keeping the eventual pip install graphik instruction for after the first release (per RELEASING.md).

No source changes — documentation only.

Triage note

Repo had zero open issues and zero open PRs at the start of this cycle. A full triage sweep (README, RELEASING.md, pyproject.toml, LICENSE, _version.py, CI workflows, test suite) found the version sources already in sync and test coverage already comprehensive for the current public API — the only verifiable drift found was this installation instruction. This cycle's entire (empty) backlog is therefore accounted for by this one finding; no issues were deferred.

Test plan

• python3 -m py_compile src/main/python/preponderous/graphik/graphik.py — succeeds
• python3 -m pytest -q — 25 passed (headless via conftest.py's SDL dummy drivers)
• pip download graphik --no-deps — confirms no PyPI distribution exists today, supporting the doc claim
• Docs-only change; no Closes #N (no tracking issue existed for this)

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This PR description was drafted during a Gardener session (Stephenson-Software/gardener).

[dmccoystephenson]

Self-review rubric:

• Scope: PASS — only README.md changed; the diff is confined to the Installation section that motivated this PR, no unrelated formatting/rename churn.
• Tests-new: N/A — docs-only change, no new public method or behavior to cover.
• Tests-fix: N/A — not a bug fix; nothing to stash-and-run.
• Sibling structure: N/A — no new file added.
• Sibling renames: N/A — no identifier renamed.
• Docs: PASS — cross-checked against pyproject.toml, _version.py, and RELEASING.md; the claim ("not yet on PyPI") was verified empirically with pip download graphik --no-deps (no matching distribution found), and the new instructions don't conflict with RELEASING.md's stated release process.
• Issue resolution: N/A — no tracking issue existed for this; found during the triage sweep (noted in the PR body).
• manual validation: PASS — py_compile succeeds, import smoke test succeeds, pytest -q → 25 passed, all re-run against this PR's head commit (0b90b64).

Repo-specific rubric:

• camelCase: N/A — no new method.
• Backward-compat: PASS — no public Graphik member touched.
• Headless: N/A — no new tests added.
• Version sync: PASS — version.txt doesn't exist in this repo (superseded by the single-source _version.py); _version.py (0.2.0), pyproject.toml (dynamic, reads from _version.py), and README are unchanged/still consistent.
• No new deps: PASS — no import added.

Context: triage found zero open issues/PRs and a clean state everywhere else (version sources in sync, test coverage already comprehensive for the current public API). This installation-instructions gap was the one verifiable inaccuracy found in a full documentation sweep.

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drafted by Claude on behalf of Daniel Stephenson