terra-xhigh-2: support explicit inherited array composition

[dakusui]

Closes #74

Summary:

• Add opt-in inherited-array composition with $super splice and $super* pairwise-merge tokens.
• Preserve replacement-by-default, validate unresolved or reserved tokens after inheritance, and report same-kind composition errors.
• Allow unresolved $super deltas to compose associatively while rejecting every combination involving an unresolved $super* delta.
• Cover file and node inheritance, includes, pairwise behavior, errors, and the unresolved splice-delta chain in unit and autotests.

Verification:

• go test ./...
• Full autotest suite: 170 of 170 passing.