Gate JSDoc completeness on every package export

New doc-sync gate verify-export-jsdoc walks every module-level exported
name under packages/*/*/src and requires description prose everywhere,
plus @param per parameter and @returns on non-void annotated returns for
function-like exports, public class methods, properties, and accessors.
The parsing + check helpers move out of gen-cordis-catalog.ts into a
shared scripts/jsdoc.ts so 'documented' means one thing on both gated
surfaces.

Deliberate exemptions (documented in the RFC): heritage-declared class
members (the seam declaration is the doc's one home — the one checker
query in an otherwise pure-AST walk), cordis plugin-protocol slots
(name/inject/reusable/Config/apply, top-level and static), constructors,
overload implementations, declare-module augmentation bodies, and
re-export statements (checked at the defining module).

The 203 under-documented exports the gate found at adoption are filled
in this change, so the gate lands green; generated catalogs/graphs are
regenerated for the shifted line pointers.

RFC: docs/rfc/implemented/process/2026-07-06-export-surface-jsdoc-gate.md

packages/core/session/src/index.ts

@@ -153,10 +153,15 @@ export class Session {
     this.header = header ?? { version: SESSION_FORMAT_VERSION, id, createdAt: Date.now() }
   }
 
+  /**
+   * The append-only event log, exposed live by reference (readonly-typed, not
+   * a snapshot): later appends are visible through the same array.
+   */
   get events(): readonly SessionEvent[] {
     return this.log
   }
 
+  /** The next event's sequence number — always the log length (the `seq = log.length` contiguity contract). */
   get seq(): number {
     return this.log.length
   }
@@ -175,6 +180,9 @@ export class Session {
    *   declare how it joins the surface, the sole source of derived history) and
    *   rejected by the compiler for non-surface types like `turn/start` or
    *   `assistant/chunk`.
+   * @returns the logged event — its assigned `seq`/`time` plus the SNAPSHOT of
+   *   `data` that entered the log, so reading `event.data` back sees the logged
+   *   value, never the caller's still-mutable input.
    * @throws if `data` is not losslessly JSON-serializable (BigInt, function,
    *   symbol, undefined, non-finite number, circular ref, or an exotic object
    *   like Map/Set/Date). The event log is the durable source of truth, so this