perf(store): use FxHashMap/FxHashSet for traversal seen maps

[iammdzaidalam]

Closes #5213

This switches HashMap / HashSet to FxHashMap / FxHashSet in the structured clone/store path (core/runtime/src/store/from.rs and to.rs).

Problem

The store path uses standard collections for traversal bookkeeping (e.g. SeenMap, ReverseSeenMap).

These maps are keyed by internal identities (pointer-like values) and are hit a lot during deep or cyclic object traversal. Using the default hasher here is probably doing more work than needed.

Change

Replace those maps with FxHashMap / FxHashSet.

This only affects internal bookkeeping. There is no intended semantic change.

Benchmark

Ran a local benchmark using boa_cli with repeated structuredClone() on a deep cyclic object graph (10 rounds, release build).

Metric	std (main)	FxHash (this PR)	Improvement
Median	10,423 ms	6,894 ms	~33.8% faster
Min	9,231 ms	6,050 ms	~34.4% faster
Max	12,028 ms	8,806 ms	~26.7% faster

Notes

• no intended semantic change
• only touches internal traversal bookkeeping
• FxHash is already used in other perf-sensitive parts of the engine

Test plan

• cargo test -p boa_engine -p boa_runtime passes
• manually tested structured cloning on deep / cyclic graphs

[github-actions]

Test262 conformance changes

Test result	main count	PR count	difference
Total	52,963	52,963	0
Passed	50,539	50,539	0
Ignored	1,426	1,426	0
Failed	998	998	0
Panics	2	2	0
Conformance	95.42%	95.42%	0.00%

Tested main commit: 58a24587890677d94618259dfaa5f257ab5ef9c9
Tested PR commit: 6f04b02b844665dec6d3dfb1c7fe09d6a9aab7ca
Compare commits: 58a2458...6f04b02

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 59.80%. Comparing base (6ddc2b4) to head (6f04b02).
⚠️ Report is 913 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #5214       +/-   ##
===========================================
+ Coverage   47.24%   59.80%   +12.56%     
===========================================
  Files         476      582      +106     
  Lines       46892    63460    +16568     
===========================================
+ Hits        22154    37953    +15799     
- Misses      24738    25507      +769     

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