Add basis-pinned symbol and function scopes to :why

[flyingrobots]

Problem

A byte-range explanation can prove text ancestry, but symbol and function questions require Graft-derived semantic boundaries. Applying a stale semantic range to a newer rope head would make the explanation causally false.

Goal

Resolve symbol and function scopes through a basis-pinned Graft observation, then feed the resulting Jim UTF-8 range into the runtime-backed :why evidence pipeline.

Acceptance criteria

• Symbol/function observations identify their rope head, Graft projection identity, parser/materializer version, and coverage.
• The resulting UTF-8 range is accepted only when its head matches the requested :why basis.
• Stale, partial, unsupported, or unavailable semantic projections return typed obstructions.
• The underlying text ancestry still comes from the same bounded range-evidence contract used by character, line, and selection explanations.
• Human and machine-readable output distinguish derived semantic scope from authoritative text facts.

Constraints

• Graft does not become text authority.
• Jim does not silently rebase semantic coordinates.
• No UI-only or pixel-scraped evidence.

[coderabbitai]

🔗 Related PRs

#195 - Add honest why observation coordinates [merged]
#205 - Document graph-backed rope runtime gate [merged]

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