Refactor comment_location from Array to Hash

[st0012]

Convert ClassModule#comment_location from an Array of [comment, location] pairs to a Hash of { location => [comments] }.

Motivation

When a class like RDoc is documented across multiple files (lib/rdoc.rb, lib/rdoc/rubygems_hook.rb, etc.), each file contributes a comment. With the old Array of [comment, location] pairs, there was no efficient way to look up or replace comments by file. This matters for server live-reload (#1620): when a user edits lib/rdoc.rb, we need to clear that file's comment and re-parse — but preserve comments from other files in their original order. The Array structure made this fragile because removing and re-appending an entry moved it to the end, changing the order comments appeared on the rendered page.

The Hash structure solves this: { location => [comments] } allows O(1) lookup by file, and Ruby hashes preserve insertion order — replacing a key's value keeps it in position.

A secondary benefit: a class reopened in the same file now preserves all its comments:

# comment1
class A; end
# comment2
class A; end

With the old Array, the C parser had a special-case delete_if to deduplicate same-location entries, while Ruby parsers accumulated duplicates inconsistently. With the new Hash, each location maps to an array of comments — both comments are preserved and rendered, matching the cross-file behavior.

Changes

• add_comment: appends to array per location — (@comment_location[location] ||= []) << comment
• parse: uses flat_map to flatten per-location comment arrays into Document parts
• marshal_load / merge: use group_by(&:file) to reconstruct arrays from Documents
• documented?, search_snippet, from_module: updated for new value shape
• i18n/text.rb: handles Hash with array values
• C parser delete_if special case removed (hash key naturally deduplicates by location)

[matzbot]

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