api-docs/io-utils: Doxygen docs for I/O utilities, primitives, and stragglers

[CV-GPhL]

Summary

Adds Doxygen /// API documentation to 22 headers covering I/O infrastructure,
low-level primitives, and miscellaneous utilities:

I/O and filesystem:

• dirwalk.hpp — directory traversal with PDB code expansion, type-alias walkers
• fileutil.hpp — file open helpers, byte-swap utilities, CharArray buffer
• fstream.hpp — RAII file stream wrappers with UTF-8 path support
• gz.hpp — gzip stream (GzStream, MaybeGzipped)
• input.hpp — unified input source abstraction (AnyStream, FileStream, MemoryStream, BasicInput)
• glob.hpp — glob pattern matching
• logger.hpp — severity-level callback logger
• pdb_id.hpp — PDB code validation and path expansion

Utilities:

• util.hpp — 50+ string and container helpers (split, join, trim, case-insensitive compare, vector ops)
• span.hpp — span and mutable-vector-span view types
• iterator.hpp — bidirectional iterator policies (stride, indirect, unique, grouping, filter)
• fail.hpp — fail(), sys_fail(), unreachable() error utilities
• atof.hpp — fast locale-independent float parsing (fast_float wrapper)
• atox.hpp — string-to-integer and character classification helpers
• version.hpp — GEMMI_VERSION constant

Stragglers:

• addends.hpp — anomalous scattering correction addends
• bond_idx.hpp — bond index for topology queries
• dsn6.hpp — DSN6/BRIX electron density map reader
• enumstr.hpp — enum↔string conversion helpers
• sprintf.hpp — snprintf_z, sprintf_z, to_str, to_chars_z wrappers
• stats.hpp — Variance, Covariance, Correlation, DataStats
• pymol_select.hpp — PyMOL selection language compiler and atom selector

docs/api.rst updated with three new sections.

Test plan

• Doxygen build produces no warnings for documented entities
• @param names verified against actual C++ signatures
• No GEMMI_DLL macros removed
• No functional code changes
• psimpl implementation namespace not exposed in public docs (only base Node class documented)

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