feature, add h5:: attribute support to HDF5 backend

[steven-varga]

Background

h5cpp issue #258 added library-side scatter/gather dispatch infrastructure:

• h5::has_scatter<T> trait
• H5CPP_REGISTER_SCATTER(T) macro
• h5::scatter<T> / h5::gather<T> generic templates
• h5::detail::write_one_row / read_one_row helpers

The h5cpp-compiler HDF5 backend currently has zero C++ attribute support. It emits register_struct<T>() specializations for POD types only, with no handling of [[h5::name(...)]], [[h5::ignore]]", [[h5::chunk(...)]]", etc.

Goal

Add C++ attribute parsing and emission to the h5cpp-compiler HDF5 backend, reusing the proven architecture from pb branches #30/#31:

1. Source rewriter — lower [[h5::xxx(...)]] → [[clang::annotate("h5::xxx", ...)]]
2. Attribute reader — extract string/int args from AnnotateAttr
3. Consumer extension — H5TemplateCallback reads per-field attributes
4. Producer extension — H5Producer emits code respecting attributes

Attributes to Support (Tier 1)

Attribute	Effect on emitted code
[[h5::name("on_disk_name")]]	Override H5Tinsert field name
[[h5::ignore]]	Skip field — no H5Tinsert emitted
[[h5::chunk(N, M, ...)]]	Dataset chunked layout (required for VLEN)
[[h5::compress(gzip, 6)]]	Add filter to DCPL

Scatter/Gather Emission

For tier-2 types (structs with std::vector / std::string fields):

• Emit h5::generated::T_::row_t mirror struct
• Emit h5::generated::T_::compound_type() with H5Tvlen_create members
• Emit template<> h5::ds_t h5::scatter<T>(...) specialization
• Emit template<> void h5::gather<T>(...) specialization
• Emit H5CPP_REGISTER_SCATTER(T);

Acceptance Criteria

• h5_attr_translator.hpp rewrites clean [[h5::...]] syntax to clang::annotate
• h5_attr_reader.hpp extracts args from annotate attrs (reusable from pb)
• consumer.hpp skips [[h5::ignore]] fields and reads [[h5::name]] overrides
• producer_h5.hpp emits renamed fields and respects [[h5::chunk]] / [[h5::compress]]
• New test fixtures with [[h5::name]] and [[h5::ignore]] pass against golden files
• Scatter/gather test fixture emits working scatter<T> / gather<T> code

Dependencies

• h5cpp #258 (library-side scatter/gather) — merged to staging
• Reuse pb attribute infrastructure from branches backend, unify all descriptor emitters under --format dispatcher #30/feature, pb:: namespace-scoped attribute vocabulary (taxonomy Phase 1) #31 where applicable

Notes

• Branch from origin/staging (do not merge pb branches)
• One vocabulary, two backends: h5::name, h5::ignore, h5::doc, etc. mirror pb::name, pb::ignore, etc.
• C++26 migration path: same attribute names become [[=h5::name{...}]] under P3394