fix(prefab): edit prefab assets without prefab-stage save corruption (set_prefab_property)

[dehuaichendragonplus]

Why

Editing a single field on a prefab that contains UGUI layout drivers, auto-sizing TMP text, or Spine components via the open_prefab_stage → set_component_property → save_prefab_stage three-piece silently corrupts unrelated fields in the .prefab asset.

Root cause (reproduced deterministically on Unity 6000.3.13f1, a pure open+save with zero edits already corrupts):

• open_prefab_stage (PrefabStageUtility.OpenPrefab) instantiates the prefab into a live isolated scene with its own Canvas. [ExecuteAlways] edit-mode drivers then run without Play Mode: LayoutGroup/ContentSizeFitter and TMP auto-size dirty into CanvasUpdateRegistry and rebuild against the isolated canvas's default metrics, mutating RectTransform.m_SizeDelta/m_AnchoredPosition/m_LocalScale and TMP m_fontSize in memory; Spine SkeletonGraphic (which has no IsPartOfPrefabAsset guard) rewrites rects and, when an import/refresh reinitializes it, nulls its skeletonDataAsset.
• save_prefab_stage / close_prefab_stage then call PrefabUtility.SaveAsPrefabAsset(stage.prefabContentsRoot, …), a wholesale re-serialize with no per-field change tracking, which faithfully writes those transient values back to disk.

Observed on real prefabs (each pure open+save, no intended edit):

• A layout-heavy prefab: m_AnchoredPosition {155,-27} → {0,-27}, m_LocalScale 0.9885 → 1 frozen.
• A Spine prefab's skeletonDataAsset {fileID:11400000, guid:…, type:2} → {fileID:0} — runtime animation permanently lost (the {fileID:0} is bare with no guid, i.e. the field was genuinely null in memory, not a guid-reassignment churn).

There is no way to make SaveAsPrefabAsset write only the intended field, so a single-field edit through the stage is inherently unsafe on these prefabs.

What changed

New tools — set_prefab_property / set_prefab_properties (in ComponentPropertyFunctions): edit a component field (or fields) inside a .prefab asset in place, without opening a prefab stage:

AssetDatabase.LoadAssetAtPath<GameObject>(path)  →  resolve GameObject by hierarchy path
  →  resolve component by type  →  ComponentSerializer.WriteProperties(...)  →  AssetDatabase.SaveAssetIfDirty(root)

Because the prefab is never instantiated into a live scene, no layout/TMP/Spine rebuild tick runs, so only the field you set changes. Value format, [SerializeField] private support, and Object-reference handling ({"fileID":id} / {"assetPath":"Assets/…"}) are reused verbatim from set_component_property; the response echoes the post-write value (newValue / applied). Structural edits (add/reparent components) still use the stage.

Guard on save_prefab_stage / close_prefab_stage: before the wholesale save, scan the stage root for corruption-prone drivers (UGUI ILayoutController/ILayoutGroup, auto-sizing TMP text, Spine components — detected by interface/type name via reflection, no new UI/TMP/Spine assembly dependency). When present, the tool message carries a ⚠ WARNING naming the risk kinds and steering single-field edits to set_prefab_property. The save still proceeds (structural edits legitimately need it) — the corruption is just no longer silent.

Benefits

• Fixes a data-loss bug generally — not Spine-specific. Any prefab with layout/auto-size/Spine drivers is safe to single-field-edit via the new tools.
• API-correct replacement for the common "hand-edit the YAML text + AssetDatabase.ImportAsset(ForceUpdate)" workaround: SerializedObject resolves Object references ({fileID, guid, type}) correctly, which hand-written YAML routinely gets wrong.
• No silent corruption on the stage path either — the guard makes the risk visible with an actionable next step.
• Reuses existing ComponentSerializer machinery, so type coercion / private-field / object-ref behavior stays identical to set_component_property.

Compatibility

• Additive. set_prefab_property / set_prefab_properties are new tools; nothing else changes behavior.
• save_prefab_stage / close_prefab_stage keep their existing string return and success message verbatim — the warning is appended only when risk drivers are detected, so existing callers and Does.Contain("Prefab stage saved")-style checks are unaffected.
• Driver detection is reflection-by-name/interface, so the package gains no dependency on com.unity.ugui, TextMeshPro, or Spine.
• Note (documented in the tool description): any save that round-trips Unity serialization — including set_prefab_property — may upgrade a prefab authored by an older Unity/TMP version to the current serialization format. That is Unity's normal, harmless format migration and is orthogonal to this fix.

Testing & verification

• Verified on a live Unity 6000.3.13f1 editor via the MCP tools: on the exact layout-heavy Spine prefab that the three-piece corrupts (3 RectTransform fields rewritten on a no-op save), set_prefab_property produces a 2-line diff — only the target field — and save_prefab_stage now emits the ⚠ warning listing layout / Spine / TMP auto-size.
• Object-reference and multi-field edits confirmed; error paths return structured codes (PREFAB_NOT_FOUND, PREFAB_GAMEOBJECT_NOT_FOUND, COMPONENT_NOT_FOUND_ON_TARGET with an available component list, PROPERTY_REQUIRED, PROPERTIES_REQUIRED).
• 10 new EditMode tests (PrefabPropertyFunctionsTests) covering: only-target-field-changes + no-stage-opened, post-write echo, multi-field apply, root-path resolution, all error paths, and the layout-driver save warning. All pass; no regressions in the rest of the EditMode suite.

Changelog

Added under ## Unreleased (Added + Fixed).