Merge branch 'web2-todo' into feat/todo-multi-in-progress

Stack the parallel-in_progress change on the web todo display (#497): the
GUI is now the surface where several active items are visible, so the two
land as a chain rather than colliding on tool-todo at merge time.

Conflicts combined rather than resolved to one side: tool-todo keeps this
branch's parallel-allowing validation AND web2-todo's additionalProperties
unknown-key rejection, in src/index.ts and both README sides; the spec
drops web2-todo's 'two in_progress' rejection case and keeps its unknown-key
case; the two headless advanced-toolchain session fixtures keep this
branch's parallel transcripts.

packages/todo/tool-todo/README.i18n.yaml

@@ -1,6 +1,6 @@
 # Bilingual-pair consistency record (docs/i18n/README.md): the git blob hash of each
 # side as of the last confirmed-consistent state. Both languages carry equal authority;
 # after editing either side, bring the other along and re-record with:
-#   pnpm run verify-translation-pairing --write
-README.md: 2f9f35be9c1a0f0234e2c69650ade48e2e5ab199
-README.zh.md: bc94547c693d7a979ede62ea83afb89cc07ccf39
+#   pnpm run verify-translation-pairing --write packages/todo/tool-todo/README.md
+README.md: 30d4caca05ab3a0d7fa4291f55b3b03d7e7d1363
+README.zh.md: b0a471957940a16e10271887e6ba8c48390fb40a