test: add type-parameter diversity matrix across the suite

[millerjp]

Summary

Every unit test, Example, fuzz target, and BDD scenario uses SyncMap[string, int] (with one [string, *struct{n int}] cameo and one [int, int]). Consumers using SyncMap[K, *bigStruct], SyncMap[K, any], SyncMap[K, chan T], SyncMap[K, func()], SyncMap[K, [N]byte], or SyncMap[K, []byte] exercise type-assertion paths the suite never touches. 100% line coverage hides the gap.

Scope

New TestTypeParameterisations (table-driven, external package) exercising Load/Store/Range/Values for:

• struct V (value, not pointer)
• interface V with mixed dynamic types stored
• pointer V (nil and non-nil)
• channel V (comparable)
• fixed-array V ([N]byte)
• []byte V (non-comparable — must be rejected by CAS/CAD at compile time; document with a commented counter-example block)

Acceptance criteria

1. Matrix test covers all 6 parameterisations for Load/Store/Range/Values.
2. Existing tests still pass; coverage stays at 100%.
3. Any V-specific behaviour divergence (e.g. interface nil vs zero) is asserted explicitly.

Source: test-analyst agent.