fix(staged): memoize expensive render-loop functions in SessionModal

[matt2e]

Summary

• Memoize renderHashtagTokens(), parseContentSegments(), and hasXmlBlocks() results in SessionModal.svelte to avoid redundant regex/string work on every reactivity update
• Activity Monitor sampling showed these functions dominating render time: RegExp.match (40 samples), String.replace (99 samples), Map.set (17 samples), string concatenation (51 samples)
• Caches are keyed on message content strings (which are immutable once set); the hashtag token cache is additionally invalidated when hashtagItems identity changes

Test plan

• Open a session with user messages containing #hashtag tokens (e.g., #commit:abc123, #issue:456) and verify they still render as inline badges correctly
• Test messages with XML blocks (<action>, <branch-history>, <launch-context> tags) still show correctly as collapsible context cards
• Open a session with many user messages and verify the modal is more responsive when scrolling through long sessions
• Verify no visual regressions in the session modal

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