Integrate reviewed contributions and fix task regressions

[vtm9]

Integrate contributions #14, #15, #16, #20, #25, and #26 with their original commits preserved, and fix the review findings before they reach main.

Correct IPv4 scanning after invalid candidates and retain arbitrary leading zeros. Preserve identifier boundaries after digits. Replace recursive flood fill with an iterative traversal and add a large-region regression. Resolve the pirate-task conflicts while preserving required scores and the empty Kraken position.

Add missing scores, align task paths with their first tags, and repair timing, limits, math escaping, and examples. Keep the integer input proposed for fancy_number, explicitly defining negative numbers and zero. Keep the three-integer signature proposed for sum_of_two_equals_third, with valid boundary cases and elementary scoring. Support units without x1 in expanded notation, as its examples require.

New tasks: center_strings, custom_fibonacci, digit_snake_formatter, expanded_notation_array, flood_fill, game_of_life_step, insert_interval, largest_perimeter_triangle, longest_balanced_segment, longest_common_prefix, max_stock_profit, maximum_altitude, minimum_bracket_reversals, peak_performance_sum, permutation_period, primes_with_prime_digit_sum, self_dividing_numbers, string_blocks_transposition, summary_ranges, toeplitz_matrix, topological_sort_possible, town_judge, turtle_max_path_sum, valid_sudoku.

Changed existing tasks: black_mark, broadside_salvo, fancy_number, feeding_kraken, log_ip_sanitizer, rum_thirst, sum_of_two_equals_third, underscore.

Validation: make build passed all 13,842 asserts across 505 tasks, including name and score/time checks. Checked schemas, paths, published input bounds, and all examples for the 32 changed tasks. Independent checks passed for 3,000 IPv4 cases, 1,331 signed triples, and 200 flood-fill grids plus the maximum 50×50 connected region. Repository pre-commit hooks passed.