feat(L10): rung-3b runtime — per-variable delta residue replay

[hyperpolymath]

What

Refines the L10 rung-3b runtime residue (landed in #54) from a whole-state checkpoint to a per-variable delta. reverse <name> now restores only the state fields its matching reversible as <name> body actually mutated, leaving everything else intact.

Why

The #54 checkpoint cut snapshotted/restored the entire AgentInstance.state map. That has a real clobber bug: if a handler firing between the reversible as and the reverse changed some unrelated field, reverse would silently revert it too. Per-variable delta is the correct "undo just this block's effect" semantics.

How

• collect_set_fields (eval.rs): a static scan of the reversible body for set <field> targets, descending through every nested control construct (mirrors the type checker's collect_reversible_as_bindings recursion shape).
• ReversibleAs captures a before-image delta — the prior values of exactly those fields — as Holding(delta) before running the body.
• ReverseNamed writes those captured fields back (for (k,v) in delta { state.insert(k,v) }), rather than replacing the whole map; then marks the cell Spent.
• Over-approximation is harmless: a field set only on an untaken branch is captured at its current value, so writing it back is a no-op.

Test

New residue_reverse_is_per_field_delta_not_whole_checkpoint: debit arms undo touching only balance; a separate annotate handler changes the unrelated field note; reverse undo restores balance to its pre-debit value but preserves note — which a whole-checkpoint restore would have clobbered.

Verification

• 914 oo7-core lib tests pass (was 913 + this one)
• cargo fmt --check clean, clippy -D warnings exit 0
• examples/transactional_rollback.007 parse + check + run green
• just run-examples exit 0; error-line count identical to origin/main (98 — all in unrelated pre-existing examples, no regression)

Docs

Spec §11b.8, the example header, and docs/echo-residue-integration.adoc all updated from "checkpoint / whole state map" to "per-variable delta" wording. New CHANGELOG entry (the #54 historical entry is left intact).

Note: CI on this repo currently fails every job in ~2-3s with no logs — an org-level billing/runner-rejection block unrelated to the diff. Verification above is local.

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https://claude.ai/code/session_018CaSgNjNURC7ocsyjYh9We

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