#2372 design/primitives/extensions.md describes the pre-#2354 bundle-cache warning condition

Description

PR 2553 (swamp-club#2354) changed when the extension loader warns about cached bundles, but the design doc was not updated. The paragraph 'Fingerprint preservation on build failure (issue #265)' in design/primitives/extensions.md (around lines 1807-1818) now describes behaviour that no longer exists.

It says the cache fallback happens when bundleWithCache cannot regenerate a bundle, and that the warning fires on the fallback case (fromCache && newFingerprint !== catalogFingerprint).

After PR 2553:

• BundleResult is a discriminated union that says why a bundle came from cache: trusted-pulled (a pulled extension's on-disk bundle is reused on purpose and no rebundle is attempted), or rebundle-failed with expectedFailure.
• trusted-pulled keeps the stored fingerprint and logs only at debug.
• rebundle-failed, unexpected: the single warning is 'Rebundle failed for , using cached bundle: ' from bundleWithCache. The reconcile line drops to debug.
• rebundle-failed, expected (bare specifiers, no deno.json): bundleWithCache logs at debug, and the reconcile path's 'Bundle could not be regenerated ... source fingerprint preserved, will retry on next command' is the single warning.
• Fingerprint preservation is the same in all three cases.

Fix

Rewrite that paragraph to name the three cache cases and the single log line each one produces, and to say that fingerprint preservation is the same in all three. Docs-only change.

Why it was missed

The doc-impact check for swamp-club#2354 searched design/ only for the log message text, not for the identifiers being changed (fromCache, bundleWithCache, rebundleAndUpdateCatalog).