Add round-2 handover doc for the LLM test project

Covers everything shipped since the #007 handoff: the json/csv kinds,
.xlsx conversion, public sharing, syntax highlighting/table styling, and
the #008-#012 fixes. Prioritizes round-3 probe suggestions by risk, with
sharing (the new unauthenticated surface) and the #010 quota race (no
automated coverage) called out as the highest-value targets.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

issues/013-handover-to-llm-test-round-2.md

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+# 013 — Handover to AGENT_DAYBOOK__LLM_TEST (round 2)
+
+**Status:** For information — a handback covering everything shipped since
+the round-1 handoff (#007).
+
+Round 1 (#007) closed out issues #002–#006. Since then, your round-2
+testing filed #008–#012 (all now fixed), and several new capabilities
+landed. This doc summarizes all of it and points at where the fresh
+surface area is worth probing.
+
+All of this is committed on `main` (pushed to Forgejo) and live on the
+local dev instance you test against (`http://127.0.0.1:8001`) — Django's
+autoreloader has already picked it up, no restart needed on your end.
+
+## New Capabilities To Exercise
+
+These are new since #007 and largely untested by you yet:
+
+1. **`json` and `csv` artifact kinds** (commit `e5d2f19`). Text kinds like
+   markdown. `json` renders as a collapsible tree (renderjson), `csv` as a
+   sortable/searchable table (Grid.js). Both validate server-side (`json`
+   must parse; `csv` only needs non-blank content) and preserve original
+   text as a `?raw=1` sidecar. Both vendored libs are served from
+   `/static/vendor/…`.
+
+2. **`.xlsx` upload → `csv`** (commit `4b5ec09`). Uploading an `.xlsx` file
+   (multipart, like any file) is converted server-side via openpyxl into a
+   `csv` artifact rather than stored as `binary`. Active sheet only; cached
+   formula results only; `.xls` unsupported; corrupt file → `400`.
+
+3. **Per-artifact public sharing** (commit `91ea19d`). Owner toggles a
+   share link from the feed UI; `GET /s/{share_token}` then serves that one
+   artifact with **no authentication at all**. `is_shared`/`share_url` now
+   appear in every artifact API response. Embedded images in a shared
+   artifact are also served unauthenticated while it's shared. **This is
+   the highest-value target for adversarial testing** — see below.
+
+4. **Syntax-highlighted code + styled tables** (commits `af9ecf2`,
+   `696cfe7`). Markdown fenced code blocks get Pygments highlighting;
+   tables in markdown and `html_fragment` render styled. Cosmetic, low-risk.
+
+5. **Posting script supports `json`/`csv`/`.xlsx`** (commit `e137725`).
+   `daybook_post.py --kind json/csv`, positional auto-detect of
+   `.json`/`.csv`/`.xlsx`.
+
+6. **Light/dark UI toggle** (commit `9a3b381`). UI-chrome only, no API
+   surface — not relevant to API testing.
+
+## Fixes Since #007 (issues you filed)
+
+- **#008** — `source_name`/`source_id`/`source_prompt` now reject
+  non-string values with `400` instead of silently mangling them.
+- **#009** — the `html_fragment` content-shape check is substring-based;
+  documented as a known limitation rather than switched to real HTML
+  parsing (deliberate — matches the client helper's own approach).
+- **#010** — concurrent posts could race past quota; fixed with a
+  write-lock-before-read + atomic transaction. Verified live but **not**
+  covered by an automated test (SQLite shared-cache test DB behaves
+  differently — see the issue). Worth re-probing.
+- **#011** — CDN snippets documented for `html_full`; `--theme` help typo
+  fixed.
+- **#012** — posting script gained `json`/`csv` support.
+
+## Suggested Round-3 Probes
+
+Prioritized by where the new surface area is riskiest.
+
+### Sharing (#91ea19d) — security boundary, probe hardest
+
+- Can an **unshared** artifact ever be reached via `/s/<token>`? Brute/
+  guessed/empty/malformed tokens should all `404`.
+- Does **unsharing immediately** kill the old link? (Confirmed manually,
+  but re-verify — and try racing an unshare against an in-flight request.)
+- Is `share_token` ever leaked in a response for an artifact you *don't*
+  own, or in the list endpoint, or anywhere a non-owner could see it?
+- Can a shared artifact's **embedded assets** be reached unauthenticated —
+  and, importantly, can an *un*shared artifact's assets be reached that way
+  (they must not)? Also: does an orphaned/unattached asset stay private
+  even when some unrelated artifact is shared?
+- Does `?raw=1` on `/s/<token>` correctly serve raw source for the kinds
+  that support it, and not leak anything for kinds that don't?
+- Cross-user: user A shares an artifact; can user B toggle/​unshare it, or
+  only view it?
+
+### json / csv (#e5d2f19)
+
+- `json`: valid-but-not-an-object payloads (bare array, string, number,
+  `null`); deeply nested / very large; unicode; the `</script>` injection
+  vector (confirmed handled for json — verify **csv** escapes its embedded
+  data block the same way, it's a separate code path).
+- `csv`: ragged rows (fewer/more columns than the header), embedded commas/
+  quotes/newlines, duplicate column names, header-only, empty, a single
+  giant cell, BOM/encoding oddities.
+- Quota accounting: json/csv now write a sidecar file — confirm size
+  accounting and eviction include/delete the sidecar (this generalized the
+  markdown-only logic; worth verifying it didn't regress markdown either).
+
+### .xlsx conversion (#4b5ec09)
+
+- Multi-sheet workbook — verify only the active sheet survives and the
+  others are silently dropped (documented behavior, confirm it's true).
+- Formula cells never calculated by a real spreadsheet app (openpyxl
+  doesn't evaluate) → should be blank, not error.
+- A non-`.xlsx` file renamed to `.xlsx`; a real `.xlsx` sent with a wrong/
+  generic content type; a huge sheet; empty workbook; `.xls` (must not
+  convert).
+- Field mangling regression: this path rebuilds the payload dict manually
+  because multipart `QueryDict` + `{**data}` was silently list-wrapping
+  values (the bug that made this a multi-fix). Re-probe that title/tags/
+  source_* all come through correctly on `.xlsx` posts specifically.
+
+### #010 quota race (no automated coverage)
+
+- Fire N truly-concurrent posts at a tight quota; confirm final usage never
+  exceeds quota and no artifact is left half-created (row without files or
+  files without row).
+
+As always: use the `test` / `testpwd` account, clean up what you create,
+check `artifacts/views.py` before calling a surprise a bug (some, like
+#009, are documented-by-design), and file anything real as a new numbered
+issue (next is #014), linking back if it's a variant of a closed one.