Add bounded deleted-text explorer to causal gutter and :why

[flyingrobots]

Problem

CR-05 exposes receipt-backed deletion boundaries and CR-06 explains their causal support, but users cannot yet inspect the exact historical text removed at a boundary without reconstructing a whole historical buffer.

Goal

Materialize deleted historical text as a bounded, read-only projection from the retained basis head and expose it from the gutter and :why evidence panel.

Acceptance criteria

• A deletion boundary can request deleted UTF-8 text through an explicit basis head and byte range.
• The response carries coverage, byte/fact bounds, continuation posture, and supporting rewrite/diff/tick identities.
• Deleted lines render as read-only virtual rows that do not alter buffer coordinates.
• Start-of-file, middle, end-of-file, and empty-file deletions remain discoverable.
• Large or unavailable history returns a typed obstruction rather than materializing an unbounded file or inventing text.
• Human and machine-readable surfaces consume the same bounded observation.

Constraints

• The projection is not text authority.
• Restoring deleted text creates a new explicit operation; it never mutates history in place.
• No Git diff reconstruction.

[coderabbitai]

🔗 Related PRs

#195 - Add honest why observation coordinates [merged]
#205 - Document graph-backed rope runtime gate [merged]
#268 - WF-0154 - E-Brake: Observed Absurdity Fixes (cycle) [merged]
#272 - WF-0154 - E-Brake closeout: provenance naming, guard witnesses, next-cycle designs [merged]
#277 - CR-05: Render receipt-backed causal gutter evidence [open]

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