Add static factory helpers on TagValueView for constructing common shallow values without having to spell out the tag/value pair explicitly.

Examples:

TagValueView::nothing()           // {TypeTags::Nothing, 0}
TagValueView::numberInt32(int32_t)
TagValueView::numberInt64(int64_t)
TagValueView::boolean(bool)
TagValueView::null()              // {TypeTags::Null, 0}

Motivation: call sites like:

accumulatorState.reset(value::TagValueView{value::TypeTags::Nothing, 0});

are verbose and error-prone (easy to pass the wrong zero literal). Named helpers make intent clearer and eliminate the raw tag/value pair at the call site.

Only shallow (non-heap-allocated) types should get helpers here, since TagValueView is non-owning.

Part of the SPM-4577 RAII refactor epic.