feat(react): path-map and { paths, where } intercept sugar (#143)

[timkindberg]

Summary

Closes #143 (child of #141). Stack: #149 → #150 → #151 → #152.

intercept now takes a path map or { paths, where } bag, not just the function. Exact path beats where, unmatched nodes keep defaults. Handlers stay mounted components with parts.

Bag vs path-map is by value shape so a field actually named paths or where isn't misread as a bag. Function floor is unchanged. useRenderNodeRules still exported; recipes/gallery move off it in later layers.

Agent did the grind.

Test plan

• <SchemaFields intercept={{ email: EmailHint }} /> hijacks that field only
• { paths, where } — exact path wins over where
• { where: Handler } for a field named where is a path map, not a bag
• Hoisted map (new object, same handler refs) does not remount inputs
• Function intercept still works
• CI green

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