prune: malformed legacy state can escape fail-open handling

[SUaDtL]

Problem

The prune hook's nudge path is guarded, but the later growth short-circuit trusts the loaded session record:

if rec and (st.st_size - rec.get(last_pruned_size, 0)) < cfg.min_growth:
    return 0

A truthy non-dict record raises on .get(). A dict with a nonnumeric last_pruned_size raises during subtraction. The security review for #360 reproduced this with string, boolean, and list-shaped records and confirmed by blame that the gap predates that PR.

Impact

A malformed ~/.codearbiter/prune-state.json session entry can escape the hook's fail-open contract and disrupt a submit instead of being ignored safely. The new context_bytes_freed parsing in #360 is already strict and fail-soft; this issue concerns the inherited record and growth-short-circuit handling only.

Done when

• Truthy non-dict session records fail open.
• Missing, boolean, string, nonfinite, negative, and container-shaped last_pruned_size values fail open.
• Valid numeric records preserve the existing minimum-growth short-circuit.
• Regression coverage exercises hook_run at the malformed-state boundary across the shared core copies.

Scope

Reliability hardening only. Do not change prune transformations, metric classification, defaults, or nudge thresholds.