test/docs: expand gds-sim test coverage and add documentation

[rororowyourboat]

Problem

gds-sim has only 44 tests for a standalone simulation engine. It also has no documentation pages and no per-package CLAUDE.md.

Missing test coverage

• Experiment.run() parallel execution paths
• compat.adapt_policy() / adapt_suf() cadCAD signature wrapping
• _positional_count() introspection edge cases (lambdas, built-ins, partials)
• Error paths: invalid StateUpdateBlock construction, malformed policies/SUFs
• Edge cases: single-block models, empty params, very large simulations
• Results.preallocate() capacity calculations

Missing documentation

• No docs/sim/ directory or pages
• Not in mkdocs.yml nav
• Not in docs/index.md package table
• No CLAUDE.md in packages/gds-sim/

Priority

Critical — package is a liability if anyone depends on it without these.