fix(brief): keep the dev hatches out of production builds (RB-11)

BIO-012: roleInterceptor/subjectInterceptor are correctly registered
only under isDevMode(), but three hand-written fetch adapters
(reveal-bignummer, letter-preview, org-template's proefbrief) bypass
HttpClient and set X-Role/X-Subject themselves with no guard. The
readers underneath, role.ts and subject.ts, were ungated too: they
read ?role=/?subject= and wrote it into sessionStorage on any
navigation, in any build -- for ?subject= that value is a BSN, which
is exactly what SessionStore's G1 comment promises never happens.

Gate both layers: currentRole()/currentSubject() return their safe
default immediately outside isDevMode() (no query-param read, no
sessionStorage write), and the three adapters additionally wrap their
headers in isDevMode() so a production request carries neither header
at all, matching what an HttpClient request already does once the
interceptors aren't registered.

TE-002: reveal-bignummer's response-shape validation was a "Trust
boundary" a spec could only reach by stubbing globalThis.fetch.
Exported it as parseRevealed(body), matching the other 30 parse*
boundaries in the repo. Same treatment for letter-preview's
errorMessage and org-template's proefbrief error mapping (extracted
from an inline try/catch into a named, exported function first, since
it wasn't already separate).

BIO-006(a): reveal-bignummer sent X-Step-Up: 'true' unconditionally,
so the backend's step-up precondition constrained nothing. reveal()
now takes a stepUp flag; BriefStore.revealBigNummer() -- reachable
only after the UI's confirm() gesture -- is the one that supplies it,
so the literal no longer lives in the transport adapter.

BIO-006(b): documented in roles-and-access.md that drafter is also
the backend's fallback identity (StubIdentityProvider's catch-all
arm), not just the dev switcher's initial choice -- so the
least-privilege consequence of it also being the only role that may
reveal a BSN is visible.

Doc correction, same diff: roles-and-access.md's "wired only under
isDevMode()" claim was false for the three hand-written fetch paths;
it now says where the gate lives (interceptor registration and the
reader functions) so it doesn't go stale the same way again.
CLAUDE.md's dev-only claims needed no correction -- they already
noted these three calls bypass the interceptor.

Every fix has a test confirmed red by temporarily reverting the
source change and rerunning the suite before restoring it.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

apps/ssp/src/app/brief/infrastructure/letter-preview.adapter.ts

@@ -1,4 +1,4 @@
-import { Injectable } from '@angular/core';
+import { Injectable, isDevMode } from '@angular/core';
 import { Result, ok, err } from '@shared/kernel/fp';
 import { currentRole } from '@shared/infrastructure/role';
 import { currentSubject } from '@shared/infrastructure/subject';
@@ -15,7 +15,10 @@ export const PREVIEW_FAILED = $localize`:@@brief.preview.failed:De voorvertoning
  * hand-written fetch, not the `ApiClient`. That also means it bypasses `HttpClient`'s
  * `roleInterceptor` AND `subjectInterceptor`, so both `X-Role` and `X-Subject` are set
  * here explicitly (WP-74 — without `X-Subject` this always previewed
- * `DocumentStore.DemoOwner`'s letter regardless of who was actually logged in).
+ * `DocumentStore.DemoOwner`'s letter regardless of who was actually logged in). Both are
+ * dev-only identity stand-ins (`role.ts`/`subject.ts`) and are sent only under
+ * `isDevMode()`, mirroring how the interceptors themselves are only registered in dev
+ * (`app.config.ts`) — a production build sends neither header from this call (BIO-012).
  *
  * `cache: 'no-store'` (WP-74): the endpoint has no `Cache-Control`, only a CORS-driven
  * `Vary: Origin`, and its content changes at the SAME URL as the letter moves
@@ -43,7 +46,9 @@ export class LetterPreviewAdapter {
       const subject = currentSubject();
       res = await fetch(`${environment.apiBaseUrl}/api/v1/brief/preview`, {
         cache: 'no-store',
-        headers: { 'X-Role': currentRole(), ...(subject ? { 'X-Subject': subject } : {}) },
+        headers: isDevMode()
+          ? { 'X-Role': currentRole(), ...(subject ? { 'X-Subject': subject } : {}) }
+          : {},
       });
     } catch {
       return err(PREVIEW_FAILED);
@@ -53,7 +58,9 @@ export class LetterPreviewAdapter {
   }
 }
 
-async function errorMessage(res: Response): Promise<string> {
+/** Trust boundary (TE-002): maps a non-OK response to a message. Exported so a spec
+    can call it directly instead of stubbing `globalThis.fetch`. */
+export async function errorMessage(res: Response): Promise<string> {
   try {
     return problemDetail(await res.json(), PREVIEW_FAILED);
   } catch {