Bug/bridge hook gating followups

[jlarson4]

Description

• Callable hook filters – add_hook now skips gated-off hook points and warns naming them, instead of silently attaching hooks that never fire.
• MoE routing observables – MoERouterBridge fires hook_expert_weights and hook_expert_indices in HookedTransformer's [tokens, num_experts] layout, scattering top-k weights in and gathering them back so edits still reach HuggingFace.
• MoE router wiring – GPT-OSS gains the router submodule it lacked, Granite declares its reordered tuple positions, and JetMoe opts out because its 5-tuple has no top-k pair.
• MoE hook aliases – blocks.N.mlp.hook_expert_* resolves onto the router submodule, so code migrated from HookedTransformer finds the hooks under the legacy names.
• Benchmark allowlist – the mlp.hook_expert_indices and mlp.hook_expert_weights exemptions are removed now that both hooks exist.
• Encoder-decoder weight stacking – the stacking helpers walk encoder_blocks and decoder_blocks as one index space and resolve the decoder's self_attn, so all eight stacked properties work on T5 instead of raising AttributeError.
• Encoder-decoder head labels – all_head_labels emits the EL/DL scheme derived from real block counts, replacing a list that named only half the heads.
  -BERT pooler – the adapter wraps the pooler module rather than its inner Linear, so hook_out carries the post-tanh pooled [CLS] that HookedEncoder exposes as hook_pooler_out.
• Pooler component – a new BertPoolerBridge aliases hook_pooler_out onto hook_out for migrated code.
• Next-sentence prediction – to_sentence_pair_tokens and predict_next_sentence pair-tokenize two strings and own the token_type_ids plumbing that callers otherwise forget.
• Audio frame entry – encoder_output runs the audio encoder from precomputed frames, bypassing the convolutional front end while leaving the start_at_layer guard intact.
• Tautological tests removed – four adapter tests asserting n_key_value_heads against their own fixture's input are deleted.

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)

Checklist:

• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• I have not rewritten tests relating to key interfaces which would affect backward compatibility