Add Maybe.some / Maybe.none constructors with T->Maybe<T> coercion

[WalterGropius]

Summary

Adds explicit Maybe.some(value) and Maybe.none constructors, mirroring
the choice-style pattern, and lets a plain T value coerce to
Maybe.some(value) in any position that already expects Maybe<T>
(return, let-with-annotation, or argument). The existing .has/.value
member access and check maybe semantics are unchanged.

Source

Addresses B3. Reproduced against main @ 999a46b:

• fun a(x:u64)->Maybe<u64>{return x} failed with TYP003.
• return Maybe.some(x) failed with NAM003 because Maybe was not a
  visible symbol.

Changes

• native/zero-c/src/checker.c: recognize Maybe.some(value) and
  Maybe.none, type them as Maybe<T>, and add a T -> Maybe<T>
  coercion in check_expr_expected (covers EXPR_NUMBER and any other
  expression whose actual type is compatible with T). A new TYP011
  message variant fires when Maybe.none lacks an explicit
  Maybe<T> context.
• native/zero-c/src/ir.c: lower Maybe.some(literal), Maybe.none,
  and the T -> Maybe<T> integer literal coercion to the existing
  IR_VALUE_MAYBE_SCALAR_LITERAL. No new IR opcodes were added, so
  every backend (ELF64, Mach-O, COFF, AArch64 MVP) continues to work
  without changes. Non-literal Maybe.some payloads and non-scalar
  element types fall back to CGEN004 like other MVP gaps.
• native/zero-c/src/main.c: register a zero explain TYP011 entry
  covering the expanded message.
• docs/articles/language-reference.md,
  docs/articles/diagnostics.md: document the new constructors and the
  refreshed TYP011 scope.

Conformance

• conformance/native/pass/maybe-some.0: build present and absent
  Maybe via the new constructors and check .has.
• conformance/native/pass/maybe-coerce-return.0: function returning
  Maybe<u8> via plain T and null coercion.
• conformance/native/fail/maybe-bare-without-context.0: bare
  Maybe.none is rejected with TYP011.
• node conformance/run.mjs and
  node --experimental-strip-types scripts/snapshot-command-contracts.mts
  both pass locally.

Proposed CHANGELOG line

• Add Maybe.some(value) and Maybe.none constructors and accept plain T in Maybe<T> positions.

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