feat: three new styles — constellation, skyline, and metro

[hahwul]

What

Three new renderers join the seven existing styles:

• constellation — a night sky. Rank sets each star's size and glow (spiral's power taper), stars sit on a hash-jittered lattice with shuffled cell assignment (provably ≥ gap apart for any config/scale), near neighbours join into constellations via a pruned Prim spanning tree, and two depths of deterministic dust fill the dark. Glow is a shared radial gradient — no SVG filters, so the PNG path is safe.
• skyline — a city. Building height comes from a dense rank over distinct weights (outlier-proof), pinned to max_height at the top and hash-spread when everyone is equal. Each building draws one of six roof silhouettes, a wall inset, and a glazing (square/ribbon, hash-lit panes) from a position-seeded lottery — random to the eye, exact to the golden files. The section's tallest tower gets the antenna.
• metro — a transit map. Stations on a route that snakes across the wall with rounded 180° turns; each section is a line in its own colour (8-colour cycle that holds contrast on light and dark), with heavier terminus rings at both ends. role_lines: true splits a section into one line per role, named after it — and weave: true interleaves those lines' rows so the routes genuinely cross, with every vertical rail landing midway between station columns so a crossing can never touch a ring (validated: gap ≥ 2.5 × line_width).

constellation and skyline honour per-user scale (skyline in height only — the lattice stays true); metro ignores it like the other fixed lattices.

Review fixes folded in

A 10-finding adversarial review ran against this diff; nine are fixed here:

• a sparse sky (fewer people than lattice columns) no longer scatters stars past its own document
• metro sizes its document for role-line titles and the weave legend
• the turn margin funds the route's stroke width, so loops are no longer shaved at the canvas edge
• the station pitch funds a terminus ring on both sides, keeping gap the ring clearance it is documented to be
• woven rails clear station name labels (pitch widens under long names)
• the weave stagger cedes one more column per round, so line i+3 can't land its rails on line i's exact x
• the stale (mosaic, spiral, orbit) scale comment in the config schema
• the four copies of the golden-critical hash mixers are hoisted into the base Renderer
• skyline's wrap pitch derives from 2 * PAD instead of a bare 12

The tenth (O(n²) MST at ~5000+ stars) is deferred: the pipeline embeds every avatar as base64, so walls that size are bounded by fetch and file size long before this term shows up.

Docs / tests

• Per-style README sections with option tables, scale table, full config reference, CHANGELOG; showcase heroes regenerated (examples/{constellation,skyline,metro}.svg)
• 3 new spec files (goldens + property specs: no-overlap under extreme configs, height monotonicity, serpentine reversal, weave interleaving/crossing coordinates, validation refusals), and the cross-style hostile-name / list-order suites now cover all ten styles
• just check clean, 297 examples pass under --order=random