api: Stop requiring private vector helpers

[BenWestgate]

Before v1, constructing and checking reference vectors must not require underscore-prefixed imports, while unrelated implementation internals should remain private rather than being indiscriminately re-exported.

Implemented and reviewed in PR #53:

• publish the checksum specifications/u5 conversion/checksum-selection helpers needed for external vector work at their owning modules;
• document a vector-construction workflow using supported names only;
• audit production cross-module private imports and document the remaining correction/profile/wallet/Core/CLI couplings instead of renaming them cosmetically;
• keep package-level codex32.__all__ deliberate.

The reviewed follow-up also makes chars_to_u5() ASCII-case-insensitive without Unicode folding and introduces the shared private _ascii_lower helper intended to absorb #13's duplicate ASCII-only normalization.

Final integration is intentionally deferred until the overlapping runtime/API prerequisites settle: #42/#57/#46, #13, #33 and #64. #7/#51 are already in reviewability-v1; #45 is already in #42. After #64, package-level codex32.__all__ should contain 23 names; #53's module-level vector helpers do not change that count. Refresh #53 once, rerun vector/API/full CI and review threads, then apply the repository's human rewrite/squash policy to the Codex-authored follow-up before merge.

[BenWestgate]

PR #53 addresses the user-facing part of this issue: reference-vector construction no longer requires underscore-prefixed helpers, and the supported module-level API is documented. I am keeping this issue open because production still has cross-module imports of underscore-prefixed internals (correction engine, GF/profile, wallet/Core, and CLI couplings). Those should be reviewed as module-boundary cleanup rather than being implicitly closed by the vector-API PR.

[BenWestgate]

Production-import audit completed against PR #53: 48 underscore-prefixed symbols are used across 64 same-package import edges, all inside the four internal boundaries documented by #53 (correction engine, profile/artifact construction, wallet/Core, and CLI). None is required by the supported reference-vector workflow, and the package-level API remains the deliberate 24-name codex32.__all__. Renaming these internals would either publish construction/search hooks or merely remove Python’s private-name signal without improving the external API. #53 therefore satisfies this issue’s acceptance criteria and now closes it on merge; no separate pre-v1 rename PR is warranted.

[BenWestgate]

Audit-number clarification: at #53 head 7311db81, the production tree has 48 distinct private (module, symbol) pairs across 64 same-package import occurrences, representing 47 unique private symbol spellings. The documented vector example runs without private imports, and codex32.__all__ remains exactly 24 names. This does not change the disposition: #53 satisfies the external API requirement; the remaining private imports are internal subsystem boundaries documented in docs/developer/api.md.