docs: refine 92 stub entries with hand-written signatures + examples

Replaces auto-generated "see omc_help" stubs with real one-line
descriptions + accurate signatures + worked examples for the most
common builtins across:

  math:    abs, acos, asin, atan, atan2, ceil, floor, round, trunc,
           sign, min, max, clamp, gcd, lcm, mod_pow, bit_count,
           bit_length, digit_sum, digit_count, fnv1a_hash,
           pow, sqrt, log, log2, log10, exp, sin, cos, tan

  arrays:  arr_all, arr_any, arr_chunk, arr_contains, arr_count,
           arr_cumsum, arr_diff, arr_drop, arr_take, arr_enumerate,
           arr_find, arr_first, arr_last, arr_flatten,
           arr_from_range, arr_gcd, arr_geometric_mean,
           arr_harmonic_mean, arr_index_of, arr_is_sorted, arr_join,
           arr_max(_float), arr_median, arr_min, arr_norm, arr_ones,
           arr_partition_by, arr_product, arr_range, arr_reduce,
           arr_repeat, arr_sort_int, arr_sum, arr_sum_sq, arr_unique,
           arr_unique_count, arr_window, arr_zeros, arr_zip,
           arr_avg_distance

  dicts:   dict_new, dict_clear, dict_del, dict_get_or, dict_has,
           dict_items, dict_keys, dict_len, dict_merge, dict_pop,
           dict_size, dict_values

  strings: str_count, str_index_of, str_lower, str_upper, str_replace,
           str_starts_with, str_ends_with, str_trim, str_chars,
           str_repeat

  regex:   re_find, re_split

  core:    type_of, len, print, input, exit

Lookup now uses .rev().find() so later entries override earlier ones —
refinements append to the end of BUILTINS and the stub gets shadowed
without needing to delete it. Stubs remain for everything not yet
hand-tuned (still useful as discoverability surface).

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

Author: RandomCoder-lab