Amend CB-EV-0001; add CB-WP-0002 for cost accounting

AM-4: measured what each remediation option actually buys, rather than
leaving one recommendation unquantified. serde_yaml optional removes 6
crates, not 5 — ryu belongs to that group, since serde_json now uses
zmij for floats. Full ladder: -6 to 27, serde_json -4 more to 23,
inlining SHA-256 -8 to 19, inlining ChaCha12 -4 to 15. Only
reimplementing a primitive gets under 20, so the target is unreachable
without undoing K5/K7.

Also records that crate count compares badly across ecosystems, and
offers the alternative the count is a proxy for: 307,317 lines of
third-party source under audit against 3,398 of our own.

AM-12: corrected from "uncomputable" to measured. The refusal to
estimate was right; the claim that no instrument existed was wrong.
Session transcripts carry exact per-message usage including the cache
breakdown. This session cost $248.46 at Fable 5 rates, of which 53% is
cache reads — cost is driven by context size times turn count, not by
output volume. What is still missing is per-task attribution, since
nothing marks task boundaries in a transcript.

CB-WP-0002 makes cost measurable and attributable: survey the
instruments, decide the attribution model by ADR, spec metrics that
include cost composition rather than a bare total, build a collector
whose positive control refuses to emit unreconciled numbers, and prove
it by answering a question that could not be answered before.

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

evidence/CB-EV-0001-game-kernel.md

@@ -1,7 +1,9 @@
 # CB-EV-0001 — GROUND game kernel: acceptance evidence
 
 Status: **T08 complete, with one acceptance metric not met (AM-4).**
-Recorded: 2026-07-31
+Recorded: 2026-07-31. Amended 2026-07-31 — §4 gains measured savings per
+remediation option, and §5 corrects AM-12 from "uncomputable" to
+measured-at-session-level; see CB-WP-0002.
 Workplan: CB-WP-0001, task T08
 Spec: `specs/GameKernel.md` §4 (AM-1..AM-12)
 Baseline: `research/CB-RES-0001-game-kernel.md`, measurements in
@@ -23,7 +25,7 @@ Machine: WSL2, Linux 6.18.33.2-microsoft-standard-WSL2, rustc 1.97.1,
 | AM-8 lint | fmt + clippy clean | clean, `-D warnings` | **met** |
 | AM-10 foreign types | zero `HashMap`/`HashSet` | 0 | **met** |
 | AM-11 impl pairs | null + reference per port | 1 of 1 (`KernelRng`) | **met, narrow** |
-| AM-12 cost log | present | §5 | **met** |
+| AM-12 cost | per-task USD | $248.46 session; per-task pending | **partial** |
 
 AM-2, AM-3, AM-5 and AM-9 are not reported: see §6.
 
@@ -121,8 +123,8 @@ Attribution:
 |---|---|---|
 | `sha2` (K7 state hashing) | sha2, digest, block-buffer, crypto-common, generic-array, typenum, cpufeatures, cfg-if | 8 |
 | serde derive chain | serde_derive, proc-macro2, quote, syn, unicode-ident | 5 |
-| serde runtime + json | serde, serde_core, serde_json, itoa, ryu, memchr, zmij | 7 |
-| `serde_yaml` (scenario files only) | serde_yaml, unsafe-libyaml, indexmap, hashbrown, equivalent | 5 |
+| serde runtime + json | serde, serde_core, serde_json, itoa, memchr, zmij | 6 |
+| `serde_yaml` (scenario files only) | serde_yaml, unsafe-libyaml, indexmap, hashbrown, equivalent, ryu | 6 |
 | `rand_chacha` (K5 seeded RNG) | rand_chacha, rand_core, ppv-lite86, zerocopy | 4 |
 | Clay-Borg crates | cb-kernel, cb-events, cb-game-runtime, games-ground | 4 |
 
@@ -130,20 +132,31 @@ The honest options, in order of preference:
 
 1. **Make `serde_yaml` optional** behind a `scenarios` feature. YAML is
    a test-and-tooling concern; a shipped game runtime does not need it.
-   Removes 5 crates from the default build for no loss of capability.
+   Removes 6 crates from the default build for no loss of capability
+   (`ryu` belongs to this group, not to serde_json, which uses `zmij`
+   for floats — corrected after measuring the reverse-dependency graph).
    This is the one to do first, and it improves D4 optionality as well
    as D2.
-2. **Revisit the target.** ≤20 was set before the K5/K7 contracts named
-   ChaCha and SHA-256. Those two contracts cost 12 crates between them
-   and are load-bearing for determinism. A target that a spec's own
-   contracts make unreachable is a bad target.
+2. **Revisit the target, and what it measures.** Measured savings per
+   option: serde_yaml optional −6 (→27); replacing serde_json −4 more
+   (→23); inlining SHA-256 −8 (→19); inlining ChaCha12 −4 (→15). **Only
+   reimplementing SHA-256 or ChaCha gets under 20**, so the target is
+   unreachable without undoing K5/K7.
+
+   Crate count also compares badly across ecosystems: Rust splits
+   crates far more finely than npm, so "33 vs 120 npm packages" flatters
+   us. The measurable thing crate count proxies for is third-party
+   source under audit: **307,317 lines** across all five groups, against
+   3,398 of our own. Retargeting AM-4 on audited third-party LOC, split
+   into shipped-runtime and dev-toolchain, measures the real concern and
+   cannot be gamed by crate granularity.
 
 What we are **not** doing: hand-rolling SHA-256 or ChaCha to win a
 dependency count. That trades an auditable, well-tested primitive for a
 number on a scoreboard.
 
-This is a T09 input: either the metric moves for a stated reason, or
-option 1 lands and the remainder is justified.
+Carried forward as an open decision (see the note at the head of this
+file): AM-4 is re-measured once the option is chosen.
 
 ## 5. Cost log (AM-12)
 
@@ -152,14 +165,30 @@ Per `specs/MetricsAndScenarios.md` §1a. Model: Claude Fable 5, at
 
 | Task | Model | Iterations | Notes |
 |---|---|---|---|
-| T08 | claude-fable-5 | 6 code iterations + benchmarks | Token counts not captured per iteration; see limitation below |
+| CB-WP-0001 (whole session, T01–T09) | claude-fable-5 | 6 T08 code iterations + benchmarks | $248.46 measured; per-task split pending CB-WP-0002 |
 
-**Limitation, stated rather than fabricated:** exact per-task token
-counts were not instrumented during T08, so the USD figure the metric
-asks for cannot be computed honestly from this run. Recording an
-estimate here would defeat the purpose of the metric. T09 should either
-wire real token accounting into the loop or drop M-D2-CST as
-unmeasurable in this setup.
+**Correction (2026-07-31).** This section originally recorded AM-12 as
+*uncomputable*. That was wrong. The declining to estimate was right; the
+conclusion that no instrument existed was not. Every session transcript
+(`~/.claude/projects/<slug>/<session>.jsonl`) carries exact per-message
+`usage` including the cache breakdown. Read for this session:
+
+| Component | Tokens | Cost (Fable 5) |
+|---|---|---|
+| Output | 585,528 | $29.28 |
+| Cache read | 131,863,164 | $131.86 |
+| Cache write (1h) | 4,365,668 | $87.31 |
+| Input | 1,090 | $0.01 |
+| **Session total** | | **$248.46** (~$124 on Opus 5) |
+
+**53% of the cost is cache reads**, not output. Cost in an agentic loop
+is driven by context size × turn count, which no "tokens per task"
+metric would have surfaced.
+
+Still missing is *attribution*: this is a whole-session figure, not a
+per-task one, because nothing marks task boundaries in the transcript.
+That is what CB-WP-0002 is for. The AM-12 row above should be read as
+"session-level cost measured; per-task attribution pending CB-WP-0002".
 
 ## 6. Metrics not reported