Moved infix::calculate() from writers.cc, fixed problem with Ref lookups

[matteB10]

1. Moved the Rewriter infix::calculate into its own file, calculate.cc to decouple and keep code coverage measures separate.
2. Updated the exists and can_replace action functions. We discovered that the old can_replace failed to find any replaceable identifers during fuzzing, including when using --gen-bound to generate variables with symbol‑table bindings. The issue turned out to be that the rule called can_replace on the Ref node instead of its Ident child. Since generated Ref nodes don’t always share the same location as their Ident, the rule never matched. The updated version fixes this by applying the check directly to the Ident node.

[Copilot]

Pull request overview

This PR refactors the infix sample’s infix::calculate() rewriter out of writers.cc into a dedicated translation unit (calculate.cc) and fixes symbol-table lookup predicates so Ref replacement checks are applied to the Ident node (avoiding location-mismatch issues seen during fuzzing).

Changes:

• Moved the infix::calculate() rewriter implementation (and its passes) from writers.cc into the new calculate.cc.
• Updated exists / can_replace usage so the predicate is applied to the Ident child instead of the Ref node.
• Updated the infix sample build targets to compile and link the new calculate.cc file.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
samples/infix/writers.cc	Removes calculate() and the maths/cleanup pass implementation from the writer compilation unit.
samples/infix/CMakeLists.txt	Adds calculate.cc to both infix and infix_trieste targets.
samples/infix/calculate.cc	Introduces the extracted calculate() rewriter and updates the lookup predicates to operate on Ident.

Suppressed comments (1)

samples/infix/calculate.cc:191

• wf_to_file is defined in this file but never referenced. Since the infix sample targets compile with -Werror, this unused variable is likely to break the build. It also duplicates the wf_to_file already used by to_file() in writers.cc, so it can be removed from calculate.cc.

  const auto wf_to_file =
    infix::wf
    | (Top <<= File)
    | (File <<= Path * Calculation)
    ;

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[Matthew Parkinson (mjp41)]

LGTM, can you fix the copilot comments please.