feat: add client-side trace upload with prefix analysis

[stmatengss]

Summary

• Add a Trace Analyzer card to the web calculator that lets users upload .jsonl trace files directly in the browser and get instant prefix sharing analysis without requiring the Python CLI
• Parses JSONL traces line-by-line with progress feedback, builds a prefix trie to compute content upper bound hit rate and shared prefix length distribution
• Auto-suggests heuristic calculator parameters (concurrent_agents, shared_prefix_tokens, avg_new_tokens_per_turn, avg_turns_per_session, private_window_tokens) derived from trace statistics, with an "Apply to Calculator" button

Features

Trace Statistics

• Request count, time span, request rate (req/s)
• Token distributions: avg, min, max, median, p95 tokens per request
• Total unique blocks

Prefix Sharing Analysis

• Builds a prefix trie from all hash_ids arrays
• Computes content upper bound hit rate = reusable blocks / total blocks
• Shows what % of requests share prefix blocks with other requests
• Interactive histogram of shared prefix length distribution
• Most common prefix lengths (median, average)

Auto-Suggest Parameters

• Maps trace analysis to heuristic calculator parameters
• concurrent_agents estimated from request rate and session length
• shared_prefix_tokens = median shared prefix length * block_size
• avg_new_tokens_per_turn = median unique (non-shared) tokens per request
• avg_turns_per_session estimated via timestamp gap clustering
• private_window_tokens = p95 request length * block_size

UI

• Drag-and-drop upload zone with dashed border styling
• Progress bar during file read and parse phases
• Clean results display with stat boxes, histogram, and parameter table
• "Apply to Calculator" button fills the heuristic form and recalculates
• "Clear Trace" button to reset and upload a different file

Test plan

• node tests/validate_js_parity.mjs passes all 178 checks (no regression to existing calculator logic)
• JavaScript syntax validated via new Function() parsing
• HTML parsing validated
• Manual: upload a JSONL trace file (e.g., Mooncake trace) and verify statistics display correctly
• Manual: click "Apply to Calculator" and verify heuristic form fields populate
• Manual: verify drag-and-drop works
• Manual: verify "Clear Trace" resets the upload zone

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