chore(performance): measure initial bytes of the built web app

Add tools/performance/measure-initial-bytes.mjs, which reads the built
dist/apps/web/index.html and sums index.html plus every same-origin
script, stylesheet and modulepreload chunk it references. The value is
the J1 ("launch to usable") counter renderer.initialBytes and matches
the "Initial total" line of the Angular build output. Missing referenced
files fail the measurement instead of counting as zero bytes.

The script has node --test coverage over synthetic build output, its own
Nx project (performance-tools, Tier B in the coverage policy), root
scripts perf:initial-bytes / perf:tools:test, and a new
docs/architecture/performance-journeys.md contract linked from the
validation map, the context map and the README.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

tools/coverage/coverage-policy.json

@@ -317,6 +317,12 @@
                 "validationCommand": "pnpm nx test eslint-tools",
                 "reason": "Node tests assert the committed max-lines baseline still matches what the generator produces; the scripts are lint tooling, not shipped source, and percentage coverage over a generated list would not mean anything."
             },
+            {
+                "name": "performance-tools",
+                "root": "tools/performance",
+                "validationCommand": "pnpm nx test performance-tools",
+                "reason": "Node tests validate the initial-bytes measurement over synthetic build output; the scripts are performance tooling, not shipped source."
+            },
             {
                 "name": "shared-marketing-fixtures",
                 "root": "libs/shared/marketing-fixtures",