fix(check): resolve callee dispatch and optional weights

[zhen8838]

Why

• analyze --dim rejected a call to a dispatch prototype even when the caller's dimension bindings selected exactly one variant.
• check required --weights even when a run reached no weights, while a declaration-based gate could not identify weights reached through child modules.

What

• Select dispatching callees with variant_for during dimension specialization, recursively rebuild the chosen body, and preserve specialization provenance.
• Make omitted --weights an empty resource so unused weights need no source and reached weights fail at their first read with the declaring module and weight name.
• Replace stale tests and examples with coverage for both callee variants, nested dispatch, missing dimensions, and the four reached/unreached weight-source cases.

Contract

• HIR specialization now resolves a callee dispatch from the caller's dimensions, rebuilds affected call targets, and records provenance; evaluation continues to dispatch from concrete argument shapes.
• --weights is optional. Without it, a reached weight is rejected at first use and names its declaring Module and weight.

Risk

• Dispatch variants on a tuple-returning entry still cannot express the required return annotation (TF-2b).
• gen_model.py documents the removed specialization limit but intentionally still emits the direct attend_by_context call; switching it back to the prototype is a separate model change.