test: characterize dangling link reference scrubbing

[christian-byrne]

Pins dangling-link cleanup behavior with four focused tests. No production logic changes.

Full context for agent readers

Adds current-behavior coverage for scrubDanglingLinkRefs after winning delete_node operations with explicit and incidental link removal. It also verifies that a non-structural set_widget preserves live input/output link references, emits no link-reference mutations, and remains byte-identical when replayed.

This characterizes the graph-integrity portion of KA-4 without changing op semantics, the O(nodes) scan, or call ordering.

Verification:

• focused Vitest: 4/4
• TypeScript test compilation: green
• targeted ESLint: zero errors
• build, purity, pins, profile claims, CodeRabbit config, and corpus verification: green
• full Vitest: 850/858 passed; the 8 failures are all existing check-import-graph gate cases because the mandated Node 25.9.0 runtime is unsupported by dependency-cruiser (^22 || ^24 || >=26)

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Reviewing files that changed from the base of the PR and between 2370b7e and 2eceeb3.

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• test/scrub-dangling-link-refs.test.ts

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[benceruleanlu]

Approved. The added cases meaningfully cover dangling-reference cleanup. These are non-blocking opportunities to make the characterization more complete.

Non-blocking review findings

• suggestion: Exercise live-link preservation through an operation that actually calls scrubDanglingLinkRefs. Both delete fixtures currently remove every link, while the surviving-link case uses set_widget; a delete fixture with one removed link and one unrelated live link would pin the keptIds branch under KA-4.
• suggestion: Observe the parent input/output containers, or use observeDeep, when asserting that set_widget emits no link-reference writes. Observing only the existing child Y.Array misses an equivalent whole-value replacement of port.links.

[christian-byrne]

Follow-up PR addressing both non-blocking items from this approval: #175 (assignee christian-byrne only per quiet week; approver recorded in the PR body, not assigned or mentioned).

Full context for agent readers

• Item 1 (keptIds branch): new delete_node fixture with one removed link and one unrelated live link — asserts the dangling endpoint refs are scrubbed while the live link's endpoint refs survive.
• Item 2 (observation gap): the set_widget no-link-reference-writes case now observes the parent inputs/outputs containers via observeDeep so a whole-value replacement of port.links is caught, plus a harness-sensitivity check that performs that replacement directly and asserts the observer fires (fails under the old child-array observer).

Evidence on the PR: focused file 5/5, typecheck clean, gates pass, full suite 968 passed with only the 8 pre-existing check-import-graph environmental failures also documented on #162.

Opened against this (retired) standalone repo following the #172 post-retirement precedent while FE #16644 carries the move to ComfyUI_frontend/packages/comfy-multi-player.