Implement typed HEALPix and stellar map APIs

[VPRamon]

Documentation, license, and changelog cleanup pass applied.

Summary:

• Added missing SPDX/license headers to the new HEALPix and starlight files that lacked them.
• Documented HEALPix and starlight public facades.
• Documented public HEALPix grid, map, index, nside, ordering, and error APIs.
• Documented the private RING kernel with HEALPix theta/phi/z terminology and scientific references.
• Documented starlight builder, generated-map wrapper, CSV schema, photometry, catalogue records, provenance, brightness values, and error API.
• Added an Unreleased changelog entry for the typed HEALPix and integrated-starlight APIs.
• Public HEALPix pixelization remains based on typed Direction<F> only.

Not run locally: cargo fmt, clippy, rustdoc, and tests. CI remains source of truth.

Closes #82.

[github-actions]

Package	Line Rate	Complexity	Health
src.photometry.passbands	99%	0	✔
src	91%	0	✔
src.astro.dynamics.forces	86%	0	✔
src.pod.run	100%	0	✔
src.mission	89%	0	✔
src.formats.iers	86%	0	✔
src.coordinates.observation	97%	0	✔
src.pod.process	93%	0	✔
src.event.lunar	94%	0	✔
src.formats.ilrs	95%	0	✔
src.formats.igs	90%	0	✔
src.coordinates.transform.centers	100%	0	✔
src.coordinates.transform.centers.position	77%	0	✔
src.ephemeris.elp2000	99%	0	✔
src.ephemeris	88%	0	✔
src.astro.nutation	94%	0	✔
src.formats.spice	82%	0	✔
src.ephemeris.lagrange	85%	0	✔
src.ephemeris.jpl	94%	0	✔
src.astro.dynamics	95%	0	✔
src.bodies.aircraft	98%	0	✔
src.event.stellar	93%	0	✔
src.mission.geometry	91%	0	✔
src.formats.ccsds	90%	0	✔
src.pod.estimation	90%	0	✔
src.astro.sgp4	88%	0	✔
src.pod.qc	93%	0	✔
src.formats.rinex	79%	0	✔
src.pod.propagation	94%	0	✔
src.formats.tle.omm	96%	0	✔
src.pod	100%	0	✔
src.astro	89%	0	✔
src.coordinates	96%	0	✔
src.bodies	94%	0	✔
src.event.solar	94%	0	✔
src.coordinates.transform.providers	93%	0	✔
src.catalogs	92%	0	✔
src.formats	94%	0	✔
src.formats.adsb	85%	0	✔
src.astro.dynamics.propagation	75%	0	✔
src.spice	82%	0	✔
src.coordinates.transform.frames	94%	0	✔
src.event.altitude	89%	0	✔
src.ephemeris.vsop87	100%	0	✔
src.pod.observation	87%	0	✔
src.targets	99%	0	✔
src.coordinates.transform	94%	0	✔
src.pod.problem	95%	0	✔
src.pod.product	99%	0	✔
src.formats.vlbi	100%	0	✔
src.starlight	44%	0	❌
src.pod.force	94%	0	✔
src.atmosphere	97%	0	✔
src.astro.dynamics.gravity	72%	0	➖
src.formats.tle	94%	0	✔
src.pod.spice	88%	0	✔
src.event	93%	0	✔
src.healpix	83%	0	✔
src.event.search	95%	0	✔
src.event.azimuth	94%	0	✔
Summary	91% (31297 / 34522)	0	✔