LRS-75 Make multipart parsing more strict

[milt]

LRS-74 Apparently our clj multipart parsing allows either a crlf or just an lf. That seems wrong! Cljs is more discerning. Actually it shared some of the same issues with boundary checking, and this is addressed now.

This PR:

• Requires proper CRLF line breaks per RFC 1341
• Explicitly disallows preamble + epilogue text
• Properly verifies the presence of start + end boundaries
  • Reworked clj java.util.Scanner implementation
  • Transitioned cljs clojure.string/split-based strategy to explicit boundary mapping
• Adds an implementation note to the README
• Limits multipart body errors to a single keyword
• Removed reliance on runtime asserts

[kelvinqian00]

(As you might've seen, we should get a linter workflow for lrs at some point.)

[milt]

(As you might've seen, we should get a linter workflow for lrs at some point.)

Yep, I'll put in a ticket. I have local kondo and will just do that for now on this PR