Enumerate BinOp arms in rust_refusing_int_operator_helper (NFR burn-down)

[gunbai-bot]

Replaces the _ wildcard in extdeps.languages.rust.emit rust_refusing_int_operator_helper with one arm per remaining std.syntax BinOp variant (Eq…NullCoalesce → none). The seed mirror is regenerated locally.

Why this was reverted before, and why it can land now. An NFR burn-down run reported that this enumeration raised the //gunbc/instruments:v2-native-cli emitted-crate warning_count from 0 to 52. Suspected cause: nullary variant patterns emitted as bare identifiers, which Rust binds as catch-all variables. I could not reproduce this on main f006fbe, with the site enumerated:

• gunbc test //gunbc/instruments:v2-native-cli (local): RUSTFLAGS="-D warnings" … exit_status=0 warning_count=0. The door and refusal controls pass.
• The emitted helper renders every arm qualified (BinOp::Add => …, …, BinOp::NullCoalesce => std::option::Option::None).
• A standalone fixture matching all 14 BinOp variants also renders qualified.

The emitter's bare-name fallback (rust_variant_path_opt, Absent arm) is still a writable widening arm. It becomes a refusal in a separate PR, which also files the RFM row.

Regen: claim_executor --regen-round-cost run locally (ulimit -v 24G, -j4, CTRL_BUILD_MODE=local, private CARGO_TARGET_DIR). Round 1 changed extdeps_languages_rust_emit.rs and rebuilt 4 packages. Round 2: rebuild_packages=0. Only the installed mirror is committed.

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