docs(code-runtime-python): register the host-side open seal; note the empty-first-frame billing

The review's follow-ups on the open-seal fix: (1) the settlement note's seal
section now records the HOST-side open hold seal (openParts -> openSealed,
mirroring the child _LogStream and stray-capture seals), paired. (2) the
first-fragment guard comment notes the empty-first-frame case (bills cost + 1 =
3, establishes no hold, bounded over-charge in the safe direction). (3) a
regression case commits a SEALED open hold before the truncation marker —
verified to fail if truncateLogs drops openSealed.

packages/code-runtime/code-runtime-python/src/index.ts

@@ -1567,6 +1567,13 @@ export class PythonCodeRuntime extends CodeRuntime {
               // billed cost cost - 2 fits exactly when the walk's cost is at
               // most logBudget + 2).
               if (!logsTruncated) {
+                // An EMPTY first open frame (openParts empty AND text '') bills
+                // cost + 1 = 3 but establishes no hold (the push is skipped),
+                // so the next frame is billed as a new first fragment. Not
+                // reachable from an honest child (_LogStream.write('') returns
+                // early; flush_line pushes only non-empty pending); for a
+                // forged frame it is a bounded over-charge in the safe
+                // direction (a flood exhausts the ledger into truncation).
                 const cap = openParts.length === 0 ? logBudget - 1 : logBudget + 2
                 const cost = jsonStringCostUpTo(message.text, cap)
                 if (cost === undefined) {