Revert marginal micro-optimizations, keep only significant performance wins

[Copilot]

The fork added many performance optimizations over upstream kofrasa/mingo. Some yield 5-20x improvements; others are micro-optimizations (charCode loops, indexed for-loops, manual string parsing) that marginally help while hurting readability. This PR reverts the marginal ones.

Benchmarks (upstream → all opts → significant only)

Benchmark	Upstream	All Opts	This PR
Simple equality query (10K iter)	643ms	36ms	31ms
$elemMatch match (100K iter)	450ms	36ms	38ms
find().all() 100K docs	421ms	97ms	154ms
$match pipeline 10K docs	81ms	17ms	18ms
query.test() reuse 100K docs	944ms	182ms	197ms

~95% of gains retained. All 1,859 tests pass.

Reverted (marginal, hurt readability)

• isOperator charCode loop → back to regex
• isDigitStr charCode loop → compiled regex
• charCodeAt(0) === 0x24 → [0] === "$"
• typeOf explicit if-chain → original hash lookup
• has, normalize manual for-loops → .every(), .some()
• walk indexOf/substring → split/join
• _hash.ts indexed loops → for...of
• query.ts, Context.init Object.keys+index → Object.entries

Kept (significant wins)

• isObject inline — bypasses typeOf overhead on the hottest path
• resolve() fast paths — 1-segment and 2-segment short circuits, pre-split pathArray parameter
• $elemMatch rewrite — compiles inner Query once at build time instead of per-document
• Context.from() single-context fast path — skips allocation when only one context
• $in Set fast path — O(n+m) via Set.has() for primitive arrays
• mingoCmp number/string fast paths — skips typeOf for the two most common comparison types
• isEqual for-loops — avoids closure overhead in hot equality checks
• Native Set methods for intersection, unique, $setDifference, $setEquals, $setIsSubset
• computeExpression — operator-first check avoids iterating all keys; indexOf replaces split for prefix extraction
• Pre-split paths in processQuery, $exists

---

💡 You can make Copilot smarter by setting up custom instructions, customizing its development environment and configuring Model Context Protocol (MCP) servers. Learn more Copilot coding agent tips in the docs.