[TASK] Optimize RST parsing with pattern and instance caching

[CybotTM]

Summary

Optimizes RST parsing with instance reuse and O(1) hash set lookups for hyperlink validation.

Changes

• ExternalReferenceResolver: Add SUPPORTED_SCHEMAS_LIST and isSupportedScheme() for O(1) hash set lookup
• InlineParser: Reuse InlineLexer instance instead of creating new one per parse
• InlineLexer: Use ExternalReferenceResolver::isSupportedScheme() for URI scheme validation (~6x faster)
• LineChecker: Cache compiled regex patterns
• Buffer: Cache unindent calculations

Performance Impact

See Performance Analysis Report for detailed benchmarks.

The hash set optimization for URI schemes provides approximately 6x speedup compared to the previous regex-based approach for the 371 IANA-registered schemes.

Merge Note

Both this PR and #1287 add the same isSupportedScheme() method to ExternalReferenceResolver. When the second PR merges, the conflict is trivially resolved by keeping the existing code.

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Related PRs

PR	Description	Status
#1287	Rendering caching layer	Open, reworked after review (trivial merge conflict on ExternalReferenceResolver)
#1288	This PR - RST parsing optimizations
#1289	CLI container caching	Closed — the cache was never written
#1291	Symfony 8 compatibility	✅ Merged
#1293	ProjectNode O(1) document lookup	Independent

All PRs can be merged independently in any order.

Update: three corrections after review

The three caches in LineChecker are static and keyed by the line itself, with nothing clearing or bounding them. DirectiveRule::applies() asks about every line of every document, so they retained a copy of each distinct line for the lifetime of the process — across documents, and across projects in a long running renderer. They are bounded now: a full table is dropped, which keeps the working set cached, since documents are parsed line by line, and puts a ceiling on what is held. Behaviour is unchanged.

InlineParser::parse() reused one lexer instance, but a rule may parse content of its own while it applies — a text role parsing its argument, for example. The nested call handed the same lexer new input, discarding the token stream the outer parse was reading, and a later rollback() then indexed positions that no longer existed: a TypeError, not a wrong result. No caller in this repository nests today, so no test could have caught it; for a third party text role it was a silent contract change. The shared instance is now only handed out while it is free, and a nested parse allocates its own. The docblock warned about "race conditions" — there are no threads here, the hazard is re-entrancy, and it now says so.

Buffer::trimLines() changed the lines without resetting the unindented memo that every other mutator resets. Harmless today, because trim() leaves no indentation for unIndent() to remove, but the invariant did not hold.

Two tests come with it: one for the nested parse, which fails against the previous state of this branch, and one pinning the two scheme tables — the deprecated regex the lexer still builds from, and the list behind isSupportedScheme() — to the same content, so a scheme added to one only cannot make the lexer find a link the resolver then refuses.

Assisted by claude-code:claude-opus-5 — Session