feat(v4/spec): VBSpec Stage C — adapter library + chain suggestion / insertion

Adds cppmega_v4.spec.adapters, the auto-bridge library that closes
common shape gaps the resolver surfaces (VisualBuilderSpec.md §3.3, §6 C).

Six adapter brick kinds, each with a registered BrickShapeContract and
classified as norm_or_proj in the fusion compatibility table (so they
fuse into neighbour kernels at runtime, making them effectively free):

  - adapter_merge_heads      (B,nh,S,d) -> (B,S,nh*d)
  - adapter_split_heads      (B,S,nh*d) -> (B,nh,S,d)  — handles both
    (B,nh,S,d) and (B,S,nh,d) consumer layouts so a second transpose
    rule can finish the chain.
  - adapter_transpose_bnsd   (B,nh,S,d) -> (B,S,nh,d)
  - adapter_linear_bridge    (B,S,H_in) -> (B,S,H_out)
  - adapter_rmsnorm          RMSNorm with H params
  - adapter_residual         pure passthrough residual

Public surface (re-exported from cppmega_v4.spec):
  - ADAPTER_RULES, AdapterRule (table-driven trigger/transform/description)
  - AdapterSuggestion (one step in a planned chain)
  - suggest_adapter_chain(producer_shape, consumer_shape, *, max_steps=4):
    greedy walker that tries rules in order and reports the shortest
    chain that lands on consumer_shape, or None.
  - insert_adapter_chain(graph, producer, consumer, suggestions): splices
    the chain into a BrickGraph on the named edge, preserving edge
    order and synthesising unique node names (suffix on collision).

Tests (tests/v4/test_vbspec_stage_c.py, 31 cases):
  - registration: every adapter kind has a contract AND is norm_or_proj
  - per-rule trigger/transform unit cases (merge, split, transpose,
    bridge); no false-positives when shapes already match
  - suggest_adapter_chain: empty, single-step, two-step (split + transpose),
    no-rule-path-exists, max_steps exhaustion
  - insert_adapter_chain: noop, unknown-edge raises, single insertion,
    multi-insertion preserves order, name-collision suffix
  - system: inserted adapter fuses with linear-attn neighbour (planner
    still groups all three into one region); end-to-end resolver -> chain
    -> splice -> re-resolve flow

Full v4 regression: 702 passed / 5 skipped / 0 failed.

Author: apstenku123

cppmega_v4/spec/__init__.py

@@ -9,6 +9,13 @@
 
 from __future__ import annotations
 
+from cppmega_v4.spec.adapters import (
+    ADAPTER_RULES,
+    AdapterRule,
+    AdapterSuggestion,
+    insert_adapter_chain,
+    suggest_adapter_chain,
+)
 from cppmega_v4.spec.resolver import (
     DiagnosticSeverity,
     ResolvedBrickGraph,
@@ -26,6 +33,9 @@
 )
 
 __all__ = [
+    "ADAPTER_RULES",
+    "AdapterRule",
+    "AdapterSuggestion",
     "BrickShapeContract",
     "DiagnosticSeverity",
     "ResolveError",
@@ -34,7 +44,9 @@
     "ShapeDiagnostic",
     "ShapeExpr",
     "contract_for",
+    "insert_adapter_chain",
     "register_contract",
     "registered_kinds",
     "resolve_shapes",
+    "suggest_adapter_chain",
 ]