feat(dx): minimal install, reproducible CI, portable capture artifacts

[jeffbrennan]

Implements brief 05 (plans/improvements-2026-09/05-developer-experience.md) across all three increments.

Increment 1 — minimal install and CLI

• Base install (polars, pydantic, typer) parses, analyzes, and runs sparkparse --help/--version/parse/analyze without PySpark or Dash. capture/connect load lazily via PEP 562; common.get_spark imports PySpark inside the function; app.viz imports the dashboard at the command boundary.
• Version is derived from package metadata instead of a hardcoded string.
• timeit writes to logging/stderr, so analyze stdout stays parseable JSON.
• Log dir is a positional argument with a --log-dir alias; missing local paths raise a clear BadParameter.

Increment 2 — repeatable development

• CI runs on synchronize with concurrency cancellation and UV_FROZEN=1.
• New packaging job builds the wheel once and smoke-tests base / [viz] / [spark] installs in fresh environments (base asserts no PySpark/Dash import).
• databricks.yml wheel path parameterized through a bundle variable; just build-wheel builds it.
• README: extras, canonical CLI usage, dependency-update flow, and artifact docs; stale TODO claims corrected.

Increment 3 — portable capture artifact/report

• New sparkparse/artifact.py: directory artifacts (manifest.json + Arrow dag/combined tables) plus single-file JSON, with round-trip including typed empty frames.
• capture_context(..., artifact_path=...) persists the result (and viz report.html) on both success and failure; a write failure marks the capture partial and propagates under capture_errors="raise" without masking the workload exception.
• New sparkparse/dataset.py: server-side cached access to a raw-log dir, an artifact, or an in-memory result.
• init_dashboard(source) accepts any of those. Summary aggregates server-side (browser no longer receives the raw task table), the stage timeline is precomputed, tables paginate, and unavailable panels are listed with their reasons instead of rendering blank.

Validation

• just ci (ruff check/format, pyrefly, pytest): 447 passed, 2 skipped.
• Fresh wheel installs verified locally for base (no PySpark/Dash) and [viz]; CI covers [spark].
• New tests: test_artifact.py, test_dataset.py, test_cli.py, test_home_page.py, test_dashboard_summary.py, plus failed-capture artifact coverage in test_capture_contract.py.

Not covered

The brief's manual Databricks serverless smoke procedure (Free Edition) was not run; that remains an outstanding live-validation item.