feat: configurable case directory naming (case_naming), thread-safe signal handling

[yannrichet]

Summary

• fzr() gains a case_naming parameter (CLI --case_naming, env FZ_CASE_NAMING): "path" (default, unchanged var1=val1,var2=val2,... subdirectories), "hash" (short content hash of the variable combination), or "index" (case_<i>). "path" can exceed filesystem filename length limits (~255 chars) with many input variables; "hash"/"index" avoid that.
• With "hash"/"index", a single cases.csv manifest is written at the results root mapping each case directory to its variables. fzo() reads it back (falling back to each case's own info.txt, which always has input.<var>=<value> lines) when a directory name doesn't parse as key=val,....
• fzd() now runs its internal per-iteration fzr() calls (file-based models) with case_naming="index" by default, since algorithm-generated design points can carry many variables with long float values. cache:// matching is by .fz_hash content, not directory name, so cross-iteration cache reuse is unaffected.
• Fixed: fzr/fzd installed a SIGINT handler unconditionally, which raises ValueError when called from a non-main thread (e.g. Streamlit reruns, a ThreadPoolExecutor worker, or a background thread embedding fz). Signal handler install/restore is now skipped outside the main thread instead of raising.
• Docs updated: README.md, doc/core-functions.md, doc/INDEX.md, doc/parallel-and-caching.md, skills/fz/reference.md, NEWS.md.

Test plan

• New tests/test_case_naming.py (6 tests): path/hash/index naming, cases.csv manifest content, fzo fallback to info.txt when the manifest is missing, invalid case_naming validation, FZ_CASE_NAMING env var default.
• tests/test_skill_static.py updated (scoped a too-broad choices=[...] regex to --format specifically) and passing.
• Full local suite (matching CI's ignore list for SSH/funz-protocol tests needing local servers): 844 passed, 1 failed (pre-existing, environment-only python vs python3 PATH issue in test_debug_execution.py, reproduced on unmodified main), 82 skipped.
• Manually verified fzr/fzo round-trip for hash/index naming, including after a cache-hit (fzd-like iteration reuse), and the no-variables case.

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