feat(model): the envelope checkout, and no to Joern server mode

contributor-scan 2.2 for libpng and the 2.3 decision it exists to inform.

89,082 lines of C across 93 files, under a 2GB heap cap:

  CPG build      16.1s (25s cold)
  Joern queries  250.0s
  arbitration    122.0s (105 calls, $0.0122)
  whole scan     446s
  peak RSS       911MB
  regions        121, all arbitrated, 32 unasked
  CVE-2025-66293 out_of_scope, as the recipe declares

Memory is not the constraint a fleet runner would hit: 911MB leaves room
for the rest of the job on a 2-vCPU/7GB runner. The memory pressure seen
while building this was concurrent JVMs on one desktop, not repository
size.

**Server mode: no.** The query phase splits taint 243.3s / config 10.7s
over two invocations, and fixed overhead measured on the same CPG is ~13s
per invocation. So ~26s of the 250s is JVM startup and ~224s is CPGQL
evaluation: a warm server removes 6% of the scan and none of the part that
costs. The driver now records query seconds per kind, because a single
total cannot tell startup from evaluation and the two answer this
oppositely.

The small repository pointed the other way. vampi's 34.3s over three kinds
is roughly three startups and almost no evaluation, which made a warm JVM
look worth having; the ratio inverts with size. That inversion is why the
task was sequenced after a real repository rather than decided on a
520-line file.

On whether one whole-repository CPG is the only shape for a horizontal
fleet: no, and it is also not where the pressure is. The build
parallelises and costs 16s; the query does not shard and costs 250s, so
splitting a repository across runners divides the cheap phase -- and after
2.4 a shard boundary hides exactly the cross-module flows that matter.
Scoping to a contributor's diff cuts the expensive phase instead. 911MB
for 89k lines extrapolates to roughly 5GB at a million lines, which would
exceed a standard runner, so the ceiling is real and further out than this
checkout can locate.

Two defects recorded rather than fixed, both invisible at vampi scale:
2.9, all 26 entailed findings are the same site reported 26 times, because
regions that share a sink each emit their own; and 2.10, 22 of the 30
highest-ranked regions are libpng's `contrib/` example programs rather than
the library.

Also ignores Joern's `workspace/`, which a maintainer querying a CPG by
hand from the repository root will create.

Gates byte-identical.

Co-Authored-By: Claude Opus 5 (1M context) <[email protected]>
Claude-Session: https://claude.ai/code/session_01T7XD7x5KonMdmGRkp63EQG

.kiro/specs/contributor-scan/tasks.md

@@ -98,7 +98,7 @@
   - _Requirements: 4.2, 4.3, 4.4, 4.5_
   - _Depends: 2.1_
 
-- [ ] 2.3 Decide on Joern server mode, on 2.2's numbers
+- [x] 2.3 Decide on Joern server mode, on 2.2's numbers
   - Joern offers `--server` with `--server-host`, `--server-port` and basic auth: one warm JVM with the CPG
     loaded once, and queries in milliseconds after that. It would remove the three-to-four JVM starts
     (~100s) that remain per scan now that 1.4 has batched the queries.
@@ -115,6 +115,13 @@
     so as a port listening on a contributor's laptop.
   - Observable: a recorded decision that cites 2.2's build-time-to-query-time split. A preference does not
     close this task; a ratio does.
+  - **Decided: no.** On libpng (89k lines of C) the query phase splits taint 243.3s / config 10.7s over two
+    invocations, and fixed overhead measured on that CPG is ~13s per invocation. So ~26s of 250s is JVM
+    startup and ~224s is CPGQL evaluation: a warm server removes 6% of a 446s scan and none of the part that
+    costs. Recorded in `benchmarks/measurements/2026-09-09-repo-libpng-envelope.json`.
+  - The small repository pointed the other way -- vampi's 34.3s over three kinds is roughly three startups
+    and almost no evaluation -- and the ratio inverts with size. That inversion is precisely why this task
+    was sequenced after a real repository instead of being decided on a 520-line file.
   - _Requirements: 4.2_
   - _Depends: 2.2_
 
@@ -178,6 +185,30 @@
   - _Requirements: 8.1, 8.2_
   - _Depends: 2.6_
 
+- [ ] 2.9 One defect is one finding, however many regions reach it
+  - libpng's scan produced 26 entailed findings that are all the identical site,
+    `contrib/gregbook/wpng.c:335:main`. Many regions reach one shared sink and each emits its own finding;
+    nothing dedupes by site. A contributor would be shown the same defect twenty-six times.
+  - Invisible at vampi scale, where regions rarely share a sink, and it appeared the moment task 2.4 let a
+    region's question follow the call graph.
+  - Dedupe on the reported site, keeping the strongest rung and recording how many regions reached it --
+    "reached from 26 entry points" is useful information, twenty-six rows are not.
+  - Observable: libpng's entailed count reflects distinct sites; the artifact records both numbers.
+  - _Requirements: 5.1, 8.1_
+  - _Depends: 2.4_
+
+- [ ] 2.10 Do not spend the scan on example code
+  - 22 of libpng's 30 highest-ranked regions are under `contrib/` -- the sample programs shipped with the
+    library rather than the library. The budget and the 243s taint query go there first.
+  - The rank already comes from the entry-point mapper; what is missing is that a path can be evidence too.
+    Vendored, generated, example and test trees are not what a contributor is changing. This is also the
+    cheapest available reduction of the 243s.
+  - Do not hardcode `contrib/`: derive it, record what was excluded and why, and measure the scan with and
+    without so the exclusion is a number rather than a preference.
+  - Observable: a recorded before/after on libpng -- regions, query seconds, and entailed sites.
+  - _Requirements: 4.2, 8.1_
+  - _Depends: 2.2_
+
 ## Group 3 — Ship it
 
 - [ ] 3.1 Contributor output