Method name lookup

[skatkov]

This PR speeds up method-name lookups in RDoc and makes the method collection read-only so callers cannot accidentally invalidate the lookup cache.

What changed

Previously, each lookup scanned the whole method list. Now, lib/rdoc/code_object/context/method_list.rb maintains a lazy index mapping each name to its matching methods.

• The index is built on the first lookup.
• Adding methods indexes only the appended entries—not the entire list again.
• Removing or replacing methods invalidates the index.
• Renaming a method invalidates every index containing it, including after module promotion or shared aliases.
• Mutable name strings fall back to scanning, because their contents can change without calling name=.

The index keeps methods in list order. Class/instance filtering happens at lookup time, preserving the existing selection behavior. Attributes remain outside this index.

flowchart TD
    Lookup["Context method lookup"] --> List["Internal MethodList"]
    List --> Names{"All names frozen?"}

    Names -->|Yes| Index["Lazy name index<br/>Index appended entries only"]
    Names -->|No| Scan["Scan current method names"]

    Index --> Candidates["Matching methods in list order"]
    Scan --> Candidates
    Candidates --> Result["Return first match<br/>Apply class/instance filter if requested"]

    Add["add_method"] -.->|Extend on next lookup| Index
    Change["Removal, replacement,<br/>or method rename"] -.->|Invalidate| Index

    Reader["context.method_list"] --> Snapshot["Frozen Array snapshot<br/>Method objects remain mutable"]
    List -.->|Copy contents| Snapshot

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Public API change

Context#method_list now returns a frozen Array snapshot, rather than exposing the mutable backing collection.

Callers change membership through:

context.add_method(method)
context.remove_method(method)
context.replace_methods(methods)

Existing writers in Store loading, live-preview cleanup, visibility filtering, and class promotion/merging were migrated to these operations. replace_methods changes the documentation list only; it deliberately leaves methods_hash registrations unchanged.

This is a breaking change for callers that directly mutate method_list. Method objects themselves are not frozen, so renaming remains supported.

Parser to return frozen method names.

Ruby, C and RBS parser to forcefully return frozen strings for method names. This greatly improves speed improvements.

-> TODO: Add more information about this

Other generators

I have tested this branch with other generators - rdoc-markdown, sdoc and hanna. All seem to be working just fine with these breaking changes with no modifications required.

Open Questions

• Why does the parser produce mutable strings for method names? Speed up would be so much faster, if that was not the case. Needs investigations...
• method_list now returns frozen array, this works with Aliki and rdoc-markdown. But would that cause any issues with hanna, sdoc?

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Commit: 8648339