syntax: Add range operators .. and ..=

Context

Silk has no range value. A user who wants to walk a sequence of numbers writes a
while loop with an index variable. A range operator makes this shorter and gives a
value that a future for statement can use.

Current behavior

The lexer makes a DotDot token (packages/compiler/src/Lexer.ts:74). The parser
consumes that token in one place only: the rest pattern inside a nominal pattern
(packages/compiler/src/Parser.ts:830-831). There is no ..= token.

let mut index = 0
while index < limit {
  index = index + 1
}

Requirements

1. The lexer must make a token for ..=.
2. The parser must accept a .. b and a ..= b in expression position.
3. a .. b must exclude the value b.
4. a ..= b must include the value b.
5. Both operands must have the same integer type.
6. The range must be an ordinary value that a user can pass to a function.
7. The parser must keep the rest pattern behavior in pattern position.

Example

pub fn sum(limit: usize) -> usize {
  let mut total = usize.add(0, 0)
  // The range gives the values 0 to limit minus 1.
  let span = 0 .. limit
  return total
}

Out of scope

• A for statement. That is a separate decision.
• Range patterns in a match arm. Add those after this ticket lands.
• A range over a non-integer type.

Dependencies

None.

Acceptance criteria

• The lexer makes a ..= token.
• The parser accepts a range in expression position.
• The parser still accepts .. as the rest pattern in a nominal pattern.
• A test shows both forms in the same file.
• openspec/specs/bootstrap-syntax/spec.md:746-750 gets an amendment.

Implementation note

The conflict with the rest pattern is smaller than it looks. The parser consumes
DotDot in exactly one production, inside a nominal pattern field list, and only where
it expects a field. A range in expression position never reaches that production.
Disambiguate by grammatical position. Do not choose a different spelling.

Start with the lexer token at packages/compiler/src/Lexer.ts:74 and the expression parser alongside the rest-pattern production at packages/compiler/src/Parser.ts:830-831. Add coverage for both range forms in one file, preserving the nominal-pattern rest behavior, and amend openspec/specs/bootstrap-syntax/spec.md:746-750; done means the stated lexer, parser, integer-type, inclusivity, and ordinary-value requirements are covered.