docs(erratum): reconcile W-131 KV ceilings with implemented values

[Xuepoo]

Priority: P1 | Area: architecture | Labels: docs,P1,area:architecture | Milestone: v0.1.0

Commander decision (implemented as directed, not re-litigated): amend the W-131 KV ceiling contract to the implemented values via a dated erratum note, keeping accepted status. The implemented values are strictly tighter on size/entries; depth 16 vs 8 is the documented exception carrying Core's pre-existing JSON depth. Any future bound change is a new security-reviewed decision.

What this PR does (standalone erratum, no linked issue, no Closes):

• Adds an Erratum (2026-10-05) section after Document status in docs/development/storage-and-history-boundary.md with corrected ceilings (64 KiB total per plugin, 8 KiB per value unchanged, depth 16, 256 entries), provenance (bitty-storage src/ceiling.rs constants byte-for-byte with Core crates/bitty-runtime/src/plugin_runtime/store.rs bounds, bitty-storage CTX-0003/0004 parity), and the no-silent-change rule.
• Corrects the Per-plugin KV budget-table cell from 256 KiB total / depth 8 / 1024 nodes to 64 KiB total / depth 16 / 256 entries.
• No code changes (read-only verification only). Accepted status kept; no normative weakening.

Gates: just check green, just fmt green.