⚡ Bolt: Optimize repetitive string concatenations in HTML helpers

[WarGloom]

💡 What
Refactored several functions in src/fuzz_introspector/html_helpers.py (html_get_header, create_pfc_button, html_add_header_with_link, html_get_report_creation_tag) to replace repetitive string concatenations (+=) with direct f-string formatting and variable assignments.

🎯 Why
Repeated string concatenation using += creates significant memory reallocation overhead in Python since strings are immutable. This is particularly noticeable when rendering large HTML reports, which fuzz_introspector frequently handles.

📊 Impact
Reduces memory footprint and speeds up HTML generation times slightly by minimizing unnecessary internal string buffering and copying operations.

🔬 Measurement
Verified performance gains locally using cProfile, and confirmed functional correctness by ensuring all pytest suites and code_checks.sh (flake8/yapf) passed successfully without regressions.

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PR created automatically by Jules for task 17114143183338951668 started by @WarGloom

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