docs: document 14 new string/list built-ins in reference and spec

[escapeboy]

Summary

• Adds the 14 built-ins from feat(lang): more string and list built-ins (14 new operations) #63 (opcodes 0x8C–0x99) to the Built-in functions table in docs/reference/ax-language.md: 7 string operations (contains, starts_with, ends_with, to_upper, to_lower, trim, join) and 7 list operations (len, is_empty, head, tail, append, concat, reverse).
• Adds a new §5a Standard built-in functions section to docs/spec/ax-language-1.0.md with formal type signatures and semantics for all 18 built-ins (4 original + 14 new), grouped as String operations and List operations.
• Updates the import section in ax-language.md to note that the compiler strips fn main() -> Int stubs from inlined library sources (confirmed in tooling/src/import_resolver.rs).

Test plan

• cargo check --workspace passes (verified ✓)
• Spot-check: built-in names in the reference table match opcode names in the VM implementation from feat(lang): more string and list built-ins (14 new operations) #63

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