v0.4.0

The visualisation release. Adds four visualisation tools behind an
optional [viz] extra — three matplotlib static-plot tools
(plot_ground_track, plot_trajectory, plot_porkchop) returning PNG
images, and czml_trajectory, which exports a CZML document for a Cesium
3D client via the gmat-czml sibling. Every viz tool returns its picture
alongside the numeric summary — a PNG ImageContent or a CZML
EmbeddedResource added to, never replacing, the structured / ASCII
output — so a text-only client still gets the answer. The tools register
only when the extra is importable (pip install astrodynamics-mcp[viz]),
so the base install stays viz-free.

Added

• Visualisation tools behind an optional [viz] extra — three matplotlib
  static-plot tools and one CZML export tool, plus the additive attachment
  output channel they share. plot_ground_track renders a sub-satellite
  ground track over a lon / lat graticule; plot_trajectory renders a 2D
  or 3D orbit / transfer arc about a central body; plot_porkchop renders
  a C3 contour from an existing porkchop grid with no recompute; and
  czml_trajectory exports a trajectory as a CZML document for a Cesium
  client. PNGs come back as ImageContent, CZML as an EmbeddedResource,
  each beside the inline numeric summary that stays the default. PNG
  rendering is deterministic — the headless Agg backend, a fixed DPI, and
  stripped version / timestamp metadata chunks make repeated renders
  byte-identical within a matplotlib version, so the same call returns an
  identical image over stdio and Streamable HTTP. The slots register only
  under the [viz] install, the same gate the [gmat] and [spice]
  extras use (#131, #132, #133).
• Eval-suite visualisation coverage — a single-tool prompt for each of the
  four viz tools plus a sequential sgp4_propagate → plot_ground_track
  prompt, scored by an attachment-aware hybrid scorer that asserts an
  attachment is produced and carries its declared type (ImageContent
  versus EmbeddedResource); a requires_viz gate skips these prompts
  when the extra is absent, mirroring requires_spice / requires_gmat.
  Plus two runnable example sessions — a static-plot session and a
  CZML-export session — each shipping a markdown transcript and a Python
  script that drives the same sequence against an in-process MCP server
  (#134).
• Documentation for the v0.4 surface: a Visualisation page covering the
  plot tools, czml_trajectory, the additive attachment model
  (ImageContent / EmbeddedResource), and which clients render images
  versus CZML; tool-reference entries for all four tools; output-shaping
  and supported-clients updates; the [viz] install line; and the viz
  tools table in the README (#136).