autoresearch iter 6: reject negative cursor instead of silently using 0

parsePositiveInt reported invalidity through a bool that both call sites
discarded, and returned (false, nil) whenever Atoi succeeded on a negative
number. GetLogs therefore accepted ?cursor=-5 and served it as cursor 0
('latest') instead of the documented 400. Validity now travels through the
error result, which no caller can ignore.

.auto/autoresearch-lessons.md

@@ -0,0 +1,83 @@
+# Autoresearch lessons — Wavelet / Cordis quality run
+
+Accumulated wisdom across iterations. Read this before forming a hypothesis.
+Weight recent lessons higher: the yardstick and codebase change under us.
+
+## Lesson 1 — iterations 0-1
+**Pattern**: The project's committed gate (`golangci-lint run` with `.golangci.yml`)
+had already been driven to 0 issues by a previous run, so it could no longer
+measure anything.
+**Why it worked**: Measuring against a pinned snapshot + extra analyzers in
+`.auto/lint.ref.yaml` (hash-locked by the Guard) restored headroom and made it
+impossible to lower the number by editing the config.
+**Conditions**: Any repo whose own lint gate is already green.
+**Anti-pattern**: Optimising `tagliatelle` (325 findings) or `wrapcheck` (290).
+Those are pure cosmetics — error-message wording and tag naming. A run that
+chases them will look productive while shuffling strings.
+**Metric delta**: baseline re-established at 102 instead of a dead 0.
+
+## Lesson 2 — iterations 1-4
+**Pattern**: Triage every analyzer finding for reality before "fixing" it.
+**Why it worked**: Three buckets turned out to be false positives:
+`forcetypeassert` in `core/events.go` is guarded by `returnsErr` (the handler's
+declared last out really is `error`), and both `exhaustive` switches already
+have `default:` arms — `exhaustive` only flags them because
+`default-signifies-exhaustive` defaults to false.
+**Conditions**: Always, but especially for linters whose defaults assume a
+different project convention.
+**Anti-pattern**: Adding `if !ok { ... }` branches or empty `case:` arms that
+cannot execute. That raises the score and lowers the code.
+**Metric delta**: 3 of 16 candidate linters dropped from the plan (0 gained,
+real regressions avoided).
+
+## Lesson 3 — iterations 1, 4
+**Pattern**: Pair the metric drop with a mechanically provable defect: write the
+regression test, commit, then revert *only* the source files and require the
+test to fail (`.auto/prove_fix.sh`).
+**Why it worked**: It caught a live bug that no counter measures — a
+singleflight body capturing the first caller's request context, so one
+disconnecting browser poisoned every concurrent request for that image.
+Iteration 4 kept debt flat at 93 yet was the most valuable change so far.
+**Conditions**: Every behavioural fix. A change that survives its own revert is
+not a fix, it is a rename.
+**Anti-pattern**: Calling something "hardening" without a test that fails
+without it.
+**Metric delta**: 0 for the proven bug (kept under the fix gate), 8 for the rest.
+
+## Lesson 4 — iteration 5
+**Pattern**: Strengthen the architecture gate; it is a generator of real,
+previously invisible debt.
+**Why it worked**: `check_cordis_architecture.sh` only grepped `go func(`, so
+`go w.run()` — the shape used by four long-lived cleanup loops — passed
+silently, each one able to take down the process on a panic. Widening the
+pattern surfaced them immediately.
+**Conditions**: Whenever a gate has been green for a long time. A green gate
+proves the checks exist, not that they cover anything.
+**Anti-pattern**: Weakening `.golangci.yml` (blocked outright by the Guard via
+`check_gate_weaken.py` + a SHA lock on the yardstick).
+**Metric delta**: 4 uncovered crash-on-panic sites hardened.
+
+## Lesson 5 — iteration 3
+**Pattern**: Deduplicate by extracting the shared *classification*, not the
+shared *response*.
+**Why it worked**: Two handlers mapped upload-lookup errors with copy-pasted
+blocks that had quietly drifted (different fallback status, different synonym
+constant for the same message). `filesrv.AbortUploadRecordError` handles the
+200/400 branches, and each endpoint keeps its own fallback it can still
+justify. Deleting the orphaned `ErrInvalidUploadID` constant was part of the
+change, not extra cleanup.
+**Conditions**: Duplicated error-mapping or validation blocks in sibling handlers.
+**Anti-pattern**: Silently unifying HTTP status codes across endpoints to make a
+helper fit — that is a behaviour change wearing a refactor's clothes.
+**Metric delta**: -2.
+
+## Standing notes
+- Repo facts: backend module rooted at `backend/`, gofumpt orders a single
+  import group as `Wavelet/...` before stdlib (uppercase sorts first); new Go
+  files need the Apache license header or `scripts/update_go_license.sh --check`
+  fails the Guard.
+- Handler edits require `make swagger` (cheap: it regenerates identical docs
+  when only bodies change).
+- 26 of the 96 `//nolint` directives currently suppress nothing — dead
+  suppressions are debt with a counter (`nolint_dirs`), and removing one that
+  is still load-bearing shows up as a new finding, so the metric self-corrects.