Fixed-instant chart snapshot replay

data.json records the complete output for 2026-01-01 12:00 UTC, north-positive latitude 51.5 degrees, east-positive longitude 0 degrees, Placidus and the default Algo5 Delta T model. It contains UT1 and TT, all fourteen positions and longitude rates, four axes and twelve cusps. Node distance is null and latitude is zero.

The installed Python chart_snapshot consumer uses this same input. The Python guide and C example show how to request it; Python represents absent distance as None, and C uses CELESTIAL_PRESENT_DISTANCE. The C/Python results also record resolved Delta T and house-nutation keys and each body's position-source key; the stored numerical replay is not a model-identity registry. The house key does not describe body corrections, which remain fixed IAU 1980.

This is a chart snapshot replay example, not an independent physical-accuracy oracle; its replay_margins are acceptance thresholds, explained in Validation. astro::chart::calculate leaves sign, house-placement and aspect calculations to the consumer.

Generate the values from the repository root; the binary goes to the ignored build/ directory, which must exist:

clang++ -std=c++23 -O2 -Isrc/astro -Isrc/calendar -Isrc/util \
  docs/examples/chart_snapshot/chart_snapshot_fixture.cpp -o build/chart_snapshot_example

Run this Python block from the repository root to keep the fixture's numeric text and insert the hand-maintained replay_margins:

import json
import subprocess
from pathlib import Path

path = Path("docs/examples/chart_snapshot/data.json")
stored = json.loads(path.read_text(encoding="utf-8"))
output = subprocess.check_output(["build/chart_snapshot_example"], text=True)
computed = json.loads(output)
expected_keys = stored.keys() - {"replay_margins"}
assert computed.keys() == expected_keys, (
  f"Computed/stored field mismatch (excluding replay_margins): {computed.keys() ^ expected_keys}"
)
metadata = json.dumps(stored["replay_margins"])
updated = output.replace("{\n", '{\n  "replay_margins": ' + metadata + ",\n", 1)
assert json.loads(updated) == {**computed, "replay_margins": stored["replay_margins"]}, (
  "Fixture layout changed: replay_margins insertion did not preserve computed fields"
)
path.write_text(updated, encoding="utf-8")