Replace full AST re-transformation with per-block caching strategy.
Previously, each streaming chunk triggered processor.run() on the entire
document (12 rehype/remark plugins including KaTeX and highlight.js).
Now transforms individual MDAST nodes and caches results by position hash.
In append-only streaming mode, stable blocks are reused directly from cache,
only the unstable trailing block is re-transformed.
- Add SvelteMap FIFO cache (5000 blocks, evicts oldest 1000 on overflow)
- Add getMdastNodeHash() for MDAST node fingerprinting by position
- Add isAppendMode() to detect streaming append patterns
- Add transformMdastNode() for single-node transformation with cache lookup
- Remove stringifyProcessedNode() (dead code after refactor)
Reduces streaming complexity from O(N × transforms) to O(1) for stable blocks.
Targets 200K token contexts without UI degradation on mobile devices.
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