[tobias-tengler]

Building multipart file-map paths stages temporary segment arrays, boxed indices and builder output before producing each final path string.

This PR constructs each multipart file-map path directly into its final string with an iterative typed path walk.

Method	Version	Mean	Ratio	Allocated	Alloc Ratio
Two list uploads	main	417.962 ns	1.00	1432 B	1.00
Two list uploads	PR	360.752 ns	0.86	872 B	0.61
Two nested uploads	main	585.115 ns	1.00	812 B	1.00
Two nested uploads	PR	514.198 ns	0.88	524 B	0.65
Missing-file fallback	main	263.721 ns	1.00	656 B	1.00
Missing-file fallback	PR	263.053 ns	1.00	328 B	0.50
Single upload	main	247.757 ns	1.00	816 B	1.00
Single upload	PR	240.250 ns	0.97	672 B	0.82

[github-actions]

Patch coverage

100.0% of changed lines covered (42/42)

File	Covered	Changed	Patch %
…/src/Fusion.Execution/Transport/Http/FileEntryBuilder.cs	42	42	100.0% 🟢

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Project coverage: 59.0% (311492/527787 lines)

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