Add a reproducible README asset pipeline (emitter + renderers + SVG drift-gate)

[gistrec]

Adds the reproducible pipeline that generates the README imagery from the
library's own output — all the geo-math runs in C++ (tools/assets/emit.cpp,
reusing the examples/gps_track.cpp pipeline); the Python renderers only draw.

Stacked on #42 (readme/text-quick-wins). This PR's base is that branch,
not master, so its diff shows only the asset files. GitHub will retarget the
base to master automatically once #42 merges. No README.md change here —
wiring the generated images into the README is a deliberate follow-up (so the
gifs can be eyeballed on this PR first).

What's in it

• tools/assets/emit.cpp — C++17 GeoJSON/JSONL emitter that reuses the exact
  decode → bounds → simplify → closest_point_on_path → point_at_distance → encode
  pipeline, so every picture is drawn from real library output (single source of
  truth for the geometry).
• tools/render/{bench,hero,social,gallery}.py + tools/make-assets.sh.
• Committed assets — docs/assets/benchmarks.svg, hero-pipeline.gif,
  social-preview.png, gallery/simplify.gif (the Douglas-Peucker collapse
  animated over real Tampa map tiles), and the emitter data under
  docs/assets/data/.

Reproducibility & CI

Renders are deterministic by construction: pinned matplotlib/pillow/numpy
(+ transitive deps that touch the bytes) in tools/render/requirements.txt, and
the nondeterminism knobs are disabled — fixed svg.hashsalt, svg.fonttype=path
(glyphs embedded as outlines, so output doesn't depend on installed fonts), no
Date metadata in the SVG, and a pinned Software tag in the PNG.

.github/workflows/assets.yml rebuilds the emitter (-Werror) and re-renders
every asset on macos-latest / Python 3.13 as a smoke test (they must build
and render without error), then byte-gates only benchmarks.svg — the one
genuinely platform-independent artifact (uncompressed text, glyph outlines via
pinned fonttools, no zlib/LZW amplification). The rasters and GIFs
(hero-pipeline.gif, gallery/simplify.gif, social-preview.png) are not
byte-diffed: byte-exact matplotlib Agg raster reproduction across independently
provisioned runners is unreliable, so — like the four gallery tile stills — they
are committed by hand. Regenerate everything locally with bash tools/make-assets.sh.

Deferred (not in this PR)

The four operations-gallery stills (great-circle / snap-to-route /
point-in-polygon / encode-decode .png, #5–#8) also fetch map tiles
(staticmap/cartopy + network) and are committed by hand; gallery.py
renders them (bash tools/make-assets.sh --all) and they land in a follow-up
commit. (The animated gallery/simplify.gif above is already rendered and
included.)