This PR adds support for let configuration options (`(eq := h)`,
`+nondep`, `+usedOnly`, `+zeta`) in `do` block `let` and `have`
declarations, matching the behavior available in term-level
`let`/`have`. Configuration options are rejected with `let mut` since
they are incompatible with mutable bindings. `+postponeValue` and
`+generalize` are also rejected in `do` blocks.
Follow-up to #13250 which added the parser support. Now that stage0 is
updated, this PR replaces the backward-compat index helpers with proper
quotation patterns and implements the actual `letConfig` elaboration.
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Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>
