Format 0.2: NaN errors on present data; stricter bundle root name

[alexkjames]

Format changes (normative)

NaN errors on present data (§9, §3.8, §3.12). A present datum may now carry NaN in both err_real and err_imag, meaning the producer delivered no uncertainty estimates — the honest representation for contractor processing chains (e.g. Quantec per-receiver deliverables) that report responses without uncertainties. Rules:

• Errors are pairwise all-or-nothing: one finite and one NaN is invalid.
• A missing datum still requires NaN errors; a finite or zero error on a missing datum stays invalid.
• Present DC data requires err_imag = 0 only when errors are delivered.
• Producers must not use NaN errors as a quality flag (that is use = 0) and must not fabricate errors; consumers needing uncertainties apply their own error model or floors.

Stricter bundle root name (§2). The top-level directory name must start with [A-Za-z0-9_] — excluding ., .., hidden directories, and option-like --prefixed names — and must not end with a dot (stripped by Windows). Charset unchanged.

Rationale for both is recorded in csemx-rationale.md.

Mechanics

• Beta minor per §12: readers accept only the exact format version, so 0.1 bundles need re-export as 0.2.
• Validator: scalar reference (validate_data_row) and streamed fast path updated in lockstep; the parity test class cross-checks them, with new coverage for NaN-error present rows (including DC), reversed mixed-error rows, and the root-name rule.
• Version bumped consistently: spec title/§12, both schema copies, both validator-metadata copies, all 7 example manifests + rebuilt example.csemx.zip, template, quickstart, web status line, pyproject.toml → 0.2.0.

Verification

• 106 unit tests pass with and without optional deps; check_examples.sh --full and rebuild_example_zip.py --check pass.
• Real-world check: a Quantec-exported Quellaveco bundle with 678,974 present rows all carrying NaN errors validates clean under 0.2 (after correcting its format.version), and fails only on the error rule under 0.1.

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