Delete go_sum_edit's oracle-only free functions and move the go.sum codec to formats

[mikolalysenko]

[agent] Filed by the scheduled architecture audit routine (ecosystems and formats). Register: discussion #560 register.

Kind: refactor. Source: new finding (register E52). It is an instance of review 3.7 #9 / E35 (refactor oracles kept in production) and of 4.7 J / E20 (pure codecs outside formats/).

Problem

vendor/go_sum_edit.rs carries two implementations of the same three go.sum edits. Verified on 045d7ec:

Edit	Free pub fn (no production caller)	GoSumEditor method (used by hosted rewrite_golang)
upsert zip + /go.mod lines	upsert_module_lines	GoSumEditor::upsert_module_lines
presence check	has_module_version	GoSumEditor::has_module_version
exact prune	remove_exact_module_version_lines	GoSumEditor::remove_exact_module_version_lines

• No production callers. grep -rn over the workspace, including socket-patch-node, finds the free functions only in this file's tests. The only production user is GoSumEditor (redirect/mod.rs#L6714, #L6874, #L6932, #L6950).
• A test oracle in production code. The free functions survive as the oracle for editor_matches_the_text_transforms_step_by_step (L686): ~90 production lines kept in sync by hand, with the key-matching rule ("{module} {version} " / "{module} {version}/go.mod ") spelled six times.
• A codec in the wrong module. The file is a pure hosted codec ("Pure go.sum line edits for the hosted Go redirect"), but it lives in vendor/. patch/redirect (mod.rs, upstream/golang.rs, upstream/client.rs), vex::discover::golang and lock_inventory::golang all import it from there.

Symptoms and impact

I found no open bugs. Today any change to the upsert rule, such as #509's /go.mod-only line question, has to be made twice and kept equal through the oracle test. The risk is low.

Proposed change

1. Move the file to formats/golang/sum.rs (mechanical, its own commit): GoSumEditor, go_sum_lines, is_h1_dirhash, reinsert_lines, remove_lines, remove_module_prefix_lines, go_sum_line_cmp. Re-export it from vendor::go_sum_edit for one release only if that's needed.
2. Delete the free upsert_module_lines, has_module_version and remove_exact_module_version_lines. Retarget their unit tests at GoSumEditor (feed new(text), then compare into_string()), and replace the step-by-step oracle test with fixed expected outputs.
3. Name the key rule once: fn is_version_line(line, module, version) -> bool.

Size and scope

About 100 production lines deleted, plus about 750 moved. It touches vendor/go_sum_edit.rs (moved), vendor/mod.rs, formats/mod.rs and five import sites. Out of scope: go_mod_edit.rs (E19) and the hosted Go rewriter's logic.

Acceptance criteria

• go.sum editing has one implementation (GoSumEditor) under formats/. Nothing in patch/, vex/ or lock_inventory/ imports vendor::go_sum_edit.
• The upsert, idempotence, stale-line, sort-order, CRLF and bare-\r cases from the current tests pass against the editor.
• The hosted Go redirect tests (cargo test -p socket-patch-core --lib golang) and the e2e Go suites stay green.

Dependencies

None. It is related to E20 (codecs into formats/), and this is that row's first go child.

[mikolalysenko]

[agent] Triaged as priority:p2 (Go modules refactor). Not a duplicate, and no open PR covers it.

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