parser: add (?:...) non-capturing and (?<name>...) named capture group syntax

[robertream]

Extends the POSIX ERE parser with two non-standard but widely-used group syntaxes:

• (?:...) — non-capturing group: parsed and matched but produces no StartCapture/EndCapture transitions, so N stays smaller
• (?<name>...) — named capture group: behaves identically to an unnamed group at the engine level; name is available via ERE::group_names()

New parser variants: NonCapturingSubexpression, NamedSubexpression.
New error: InvalidGroupName (empty or unclosed name).
New method: ERE::group_names() — depth-first walk returning Vec<Option<String>> in group-number order, mirroring the simplified tree traversal.

attr-macro: add named struct field binding with #[group(N)]

Extends __compile_regex_attr to handle Fields::Named in addition to the existing Fields::Unnamed (tuple struct) path:

• Fields without #[group(N)]: matched to capture groups by field name via ERE::group_names() — field order is independent of regex group order
• Fields with #[group(N)]: bound to explicit capture group index N
• #[group(N)] attributes are stripped from the emitted struct so the compiler does not see an unknown attribute
• Out-of-bounds #[group(N)] index emits a clear compile error instead of panicking

Also adds doc comment for Regex::exec describing the [Option<&str>; N] return shape and when None is returned.

tests: add coverage for named groups, non-capturing groups, and #[group(N)]

• compile_regex_group_extensions: verifies mixed (?<name>...)/(?:...) pattern compiles as Regex<3> across all four engines (non-capturing group excluded from N, named groups count same as unnamed)
• non_capturing_group, named_capture_group_tuple_struct, named_field_struct: attr-macro smoke tests for new syntax with tuple and named structs
• compile_fail tests via trybuild: unknown_field_name and group_index_out_of_bounds confirm macro emits clear compile errors

docs: document named groups, non-capturing groups, and #[group(N)] (with doc tests)

• README: add “Named Groups and Non-Capturing Groups” section with live code examples for (?:...) and (?<name>...), plus a rust,ignore example for #[group(N)] (feature-gated, so not doc-tested by default)
• ROADMAP: mark (?:...) non-capturing groups as [x] shipped; add [x] entry for (?<name>...) named capture groups
• ere-macros: extend #[regex(...)] docs with two live doc-test examples — named struct with field-order independence, and optional named group mapping to Option<&'a str>

Validation

• cargo test --all-features

Closes #2