Expose programmatic demo manifests for non-Playwright-test flows

[jmjava]

Summary

Projects without @playwright/test specs or pytest markers still need a supported way to drive demo-function rendering (CLI/VHS or declarative Python-driven Playwright).

Changes

• manifest_from_mapping — Public wrapper around the existing manifest coercion logic. Builds a validated Manifest from the same dict shape as a *.docgen.yaml, optionally with source_path for resolving relative tape, fixtures, etc.
• docgen package exports — Re-export demo-function API (load_manifest, render, manifest_from_mapping, exit-code constants, dataclasses, …) from docgen.__init__ so consumers can from docgen import …. __version__ is aligned with pyproject.toml (0.2.0).

Usage sketch

from pathlib import Path
from docgen import manifest_from_mapping, render

m = manifest_from_mapping(
    {
        "identifier": "my-cli-fn",
        "intent": "One sentence narration.",
        "demonstration": {"kind": "cli", "tape": "demos/demo.tape"},
        "output_budget": {"duration_seconds": 30, "resolution": "1280x720"},
    },
    source_path=Path(__file__).resolve(),
)
render(m, Path("out/demo-fn"))

Tests: tests/test_demo_function.py::test_manifest_from_mapping_cli, tests/test_package_exports.py.