First-class BC object for nesting/DD layer (#53)

[awickert]

Context

The domain-decomposition / nesting layer (#53) needs to read transmission BCs off one subdomain's edge and write them as prescribed BCs on the adjacent subdomain's edge. Currently boundary-condition values live in ad-hoc instance attributes (_bc_west_values, _bc_west_norm, etc.) with no clean external interface.

Scope

Design and implement a lightweight BC object (or structured dict) that:

• Is addressable per-edge and per-condition: flex.bc["west"]["moment"], flex.bc["west"]["displacement"], etc.
• Has getters and setters so the nesting layer can write: child.bc["west"] = parent.bc["east"].as_transmission().
• Converts cleanly to/from the existing internal attributes (_bc_west_values, _bc_west_norm) so no existing code paths break.
• Exposes the resolved (post-normalize) canonical string alongside any prescribed values, so the coefficient-matrix path continues unchanged.

Design note

The object does not need to be a full class hierarchy. A thin wrapper around the existing dicts — with a stable .as_transmission() method that flips sign conventions where needed (e.g. east shear transmitted as west shear with opposite sign) — may be sufficient.

Acceptance criteria

• flex.bc["west"]["moment"] returns the prescribed moment value (or None for homogeneous BCs).
• Writing child.bc["west"] = parent.bc["east"].as_transmission() correctly wires the child subdomain.
• No change to string-BC behaviour; all existing tests pass.
• At least one integration test showing a two-subdomain nesting scenario matches the single-domain solution.

Related

Split from #52. Depends on: scalar dict BCs (done, 79e2c0c). Used by: #53.