Correct the cap comment's memory figure

The cache costs roughly 650KB at the cap, not 600KB. Drop the claim
that the cap guards against malicious memory exhaustion: the device is
airgapped and stateless, so a psbt that bloats the cache costs the user
a power cycle, not funds.

src/seedsigner/models/psbt_parser.py

@@ -34,12 +34,9 @@ class PSBTParser():
     """
 
     # Upper bound on how many levels of derivation a single parse will cache. 1000 is
-    # just slightly under a 3-of-5 multisig consolidating 200 inputs and holds the cache
+    # just slightly under a 3-of-5 multisig consolidating 200 inputs, which costs roughly
-    # to a max of about 600 kilobytes. A psbt that requires more levels will still parse
+    # 650 kilobytes. A psbt that needs more levels than that still parses correctly; it
-    # correctly, but may have to derive some levels more than once. Capping the cache at
+    # just stops getting cache hits once the cache is full.
-    # a realistic upper bound protects against a maliciously crafted psbt that would
-    # otherwise consume unbounded memory while still providing cache wins for even
-    # atypically large real-world psbts.
     MAX_CACHED_DERIVATIONS = 1000