Document public API coverage

[ChrisRackauckas-Claude]

This PR should be ignored until reviewed by @ChrisRackauckas.

What changed

• Expanded source docstrings for all exported SurrogatesBase interface APIs with signatures, interface expectations, arguments where useful, and runnable doctest examples.
• Replaced the API page blanket @autodocs block with an explicit @docs listing for the public API.
• Added a QA guard that derives the public module surface and fails when a public name lacks a source docstring or an explicit rendered docs entry.

Audit notes

• Public API found: AbstractDeterministicSurrogate, AbstractStochasticSurrogate, update!, parameters, update_hyperparameters!, hyperparameters, finite_posterior.
• No Julia public, @public, or SciMLPublic markers were present.

Validation

• timeout 3600 ~/.juliaup/bin/julia +1.10 --project=. -e 'import Runic; println("Runic ", pkgversion(Runic)); exit(Runic.main(["--check", "--diff", "."]))'\n- timeout 3600 ~/.juliaup/bin/julia +1.10 --project=. -e 'using Pkg; Pkg.test()'\n- timeout 3600 env GROUP=QA ~/.juliaup/bin/julia +1.10 --project=. test/runtests.jl\n- timeout 3600 ~/.juliaup/bin/julia +1.10 --project=docs -e 'using Pkg; Pkg.develop(PackageSpec(path=pwd())); Pkg.instantiate(); include("docs/make.jl")'\n- timeout 3600 ~/.juliaup/bin/julia +1.12 --project=. -e 'using Pkg; Pkg.test()'\n- timeout 3600 env GROUP=QA ~/.juliaup/bin/julia +1.12 --project=. -e 'using Pkg; Pkg.test()'\n\n## Process\n- Synced main from origin/main, branched to agent/public-api-docs-coverage, audited exports and public markers, updated docs/tests, ran local validation, then pushed from the authenticated user fork because upstream push permission is triage-only.