Fix BUG-018, BUG-019: StateVector dict interface and facet-autodiff docs

[knutambot]

Summary

• BUG-018: get_state/get_old_state were documented as returning a plain Dict{Int,S}, but actually return StateVector, which only supported indexing/assignment/equality. Made StateVector <: AbstractDict{Int,S} and forwarded the read-only AbstractDict interface (keys, values, pairs, iteration, haskey, get) to the internal Dict, and corrected the docstrings/docs to describe the actual type instead of promising a plain Dict.
• BUG-019: the assembler docs claimed facet routines fall back to automatic differentiation like element routines do; no such fallback exists for facets (setup_facetbuffer(::Val{true}, ...) explicitly errors, already covered by test/setup.jl:94). Corrected docs/src/Workers/Assemblers.md and the facet_routine! docstring.

Dual-review

• Codex plan review flagged that the planned StateVector methods omitted Base.get, needed for AbstractDict's generic ==/in to work symmetrically with a plain Dict (Dict == StateVector and (k=>v) in StateVector otherwise throw MethodError) — confirmed with a live repro and fixed.
• Codex final-diff review: NO_FINDINGS.

Test plan

• Pkg.test() — all green, including a new "StateVector dict interface" regression testset in test/states.jl (keys/values/pairs/iteration/haskey/get/length, checked before and after mode=:flip and revert_states!) and the existing test/setup.jl:94 facet-autodiff error check.
• test/quadpoint_evaluation.jl updated to iterate the StateVector directly instead of reaching into .vals.
• docs/make.jl — fails with 24 pre-existing @example-block errors (MethodError on work!/set_time_increment!/get_dofhandler across most tutorials/how-tos). Confirmed these reproduce identically on unmodified main (8d5af51) — same count, same locations — so they are unrelated to this change. Logged as new BUG-024 in identified_bugs.md (untracked, local work-queue file) for separate investigation; suspected cause is the unpinned [sources] git dependencies on MechanicalMaterialModels.jl/Newton.jl resolving to newer, incompatible commits.

🤖 Generated with Claude Code

https://claude.ai/code/session_01KySgpVgJ5fWR9AvPK1JQoU

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.31%. Comparing base (8d5af51) to head (419a25c).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #103      +/-   ##
==========================================
+ Coverage   97.22%   97.31%   +0.08%     
==========================================
  Files          32       32              
  Lines        1368     1376       +8     
==========================================
+ Hits         1330     1339       +9     
+ Misses         38       37       -1     

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