Define HAMT options and failure contract

[helly25]

Define the public constexpr configuration and lightweight bounded-failure vocabulary shared by all HAMT forms. The initial options expose only decisions already required by the contract: fragment width and hard maximum size.

AG;DR

• adds structural HamtOptions suitable for a non-type template argument
• validates the 4- through 7-bit fragment widths retained for measurement
• rejects zero-sized containers
• defines allocation-exhausted and max-size-exceeded failures without heavyweight status machinery
• avoids prematurely exposing unmeasured ownership and iterator strategies

Validation:

• bazel test //mbo/container:hamt_options_test
• focused pre-commit passed with local clang-tidy and the sandbox-incompatible trunk wrapper explicitly skipped; clang-format passed

This is a chained draft based on PR #448 and must not merge before its predecessors.