Display RBS type signatures in documentation

[st0012]

Summary

Extract #: inline RBS annotations from comments and display them alongside method/attribute documentation in RDoc's HTML output.

• Parse #: annotation lines from comment blocks during Prism parsing, store as type_signature on MethodAttr
• Load type signatures from sig/ directory and RBS core types via RBS::EnvironmentLoader (inline annotations take priority)
• Validate annotations through RBS::Parser with warnings for invalid syntax
• Server-side type name linking using RBS parser locations — type names link to their documentation pages
• Render type signatures in the aliki theme with a dotted separator between call_seq and type sigs
• Display type signatures in ri terminal output
• Add rbs >= 4.0.0 as a gemspec dependency, bump minimum Ruby to 3.2

Builds on #1669 (signature card design).

Screenshots

Implementation

Area	Details
Data model	type_signature accessor on MethodAttr, Marshal v4
Parser	extract_type_signature! in PrismRuby, validates via both parse_method_type and parse_type
sig/ loading	RDoc::RbsSupport.load_signatures via RBS::EnvironmentLoader, merged after store.complete
Type linking	Server-side via RbsSupport.signature_to_html using RBS parser AST locations, decoupled from generator via block
Display	Aliki theme — template, CSS; tertiary color, tight line-height, dotted separator, scaled on mobile
RI output	render_method_type_signature in ri/driver.rb
Caching	Store#type_name_lookup built once, invalidated on complete
Tests	Parser, marshal, RBS support, type linking, ri output

Follow-up work

• Merge RBS sigs with method params: Type signatures could be merged with call_seq parameters (e.g., showing typed params with default values). Deferred — significantly more complex.
• JRuby/TruffleRuby support: RBS has a C extension that cannot build on these platforms. Dropped from CI for now. Future work: make rbs an optional dependency with graceful degradation.

[matzbot]

🚀 Preview deployment available at: https://c26bfbb0.rdoc-6cd.pages.dev (commit: bfcd437)