fix(adoption): the reserved-root mint exemption — int 0 is legitimate for exactly one id

The release-holding finding from the joint gate's six real depot brains:
the adoption path's positive-int mint check false-flagged the reserved
VFS-root sentinel (the all-zeros UUID, minted int 0 BY CONSTRUCTION at
genesis on existing brains) as a corrupt mint — so every existing brain
refused log-authority adoption and stayed on the old lossy-under-power-cut
durability, defeating the release's headline crash-safety exactly where
it matters most.

The exemption, at both mint seams (the host's minter thunk and the fact
log's encoder guard): int 0 is legal iff the id is the reserved root;
zero for ANY other id remains a corrupt-mint refusal naming the reserved
exception. The codec's u64 layer already tolerated 0 — only the guards
over-refused.

Pins: adoption goes green on a brain whose VFS root carries int 0 (the
depot-brain shape, previously refused) · a non-root zero still refuses
typed at the mint seam — held at the seam itself because a full write
SELF-HEALS a poisoned zero (the index cycle re-mints before the fact is
written, which is the correct outcome and was verified in the pinning).

Gates: unit 2065/2065 · integration 830 · conformance 31/31.

src/db/factLog.ts

@@ -1325,10 +1325,16 @@ export class FactLog {
         )
       }
       const minted = this.intMinter(kind, id)
-      if (typeof minted !== 'bigint' || minted <= 0n) {
+      // Reserved-root exemption: int 0 is legitimate for exactly one id —
+      // the all-zeros VFS root, minted 0 by construction at genesis on
+      // existing brains. Zero anywhere else is a corrupt mint.
+      const isReservedRoot =
+        minted === 0n && id === '00000000-0000-0000-0000-000000000000'
+      if (typeof minted !== 'bigint' || minted < 0n || (minted === 0n && !isReservedRoot)) {
         throw new Error(
           `fact log v2: the int minter returned ${String(minted)} for ${kind} ${id} — ` +
-            `minted ints are positive bigints; refusing to write`
+            `minted ints are positive bigints (int 0 reserved for the VFS root alone); ` +
+            `refusing to write`
         )
       }
       return minted