HS parser/compiler/mock: fix 31 test failures across 7 issues

Parser:
- Relax (number? v) to v in parse-one-transition so (expr)unit works
- Add (match-kw "then") before parse-cmd-list in parse-for-cmd
- Handle "indexed by" syntax alongside "index" in for loops
- Add "indexed" to hs-keywords to prevent unit-suffix consumption

Compiler:
- Use map-indexed instead of for-each for indexed for-loops

Test generator:
- Preserve \" escapes in process_hs_val via placeholder/restore

Mock DOM:
- Coerce insertAdjacentHTML values via dom_stringify (match browser)

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

lib/hyperscript/tokenizer.sx

@@ -104,6 +104,7 @@
     "detail"
     "sender"
     "index"
+    "indexed"
     "increment"
     "decrement"
     "append"