Add SegmentedSequence range ingestion

[helly25]

Add C++20 iterator/sentinel construction and range append support to the segmented sequence. Sized inputs reserve once before construction, while single-pass inputs retain their natural prefix-on-failure semantics.

AG;DR

• adds a constrained input-iterator/sentinel constructor
• adds append_range for C++20 input ranges
• preflights sized inputs against max_size() and reserves their complete capacity
• forwards the range into iteration so rvalue ranges retain their value category and clang-tidy accepts the forwarding-reference contract
• releases acquired storage if construction throws during the constructor
• tests construction and append across segment boundaries

Validation:

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

This is a chained draft based on PR #444 and must not merge before its predecessors.