Record request preparation baseline

docs/roadmap/implementation.md

@@ -2,7 +2,7 @@
 
 Implement the remaining local work in the [performance roadmap](performance.md)
 in order. Each sequential stage is scoped for one Luna coding-agent prompt.
-Stages 1–4 are complete; Stages 5–14 remain pending. Update status and evidence
+Stages 1–5 are complete; Stages 6–14 remain pending. Update status and evidence
 as each work unit lands.
 The completed inspection plan is preserved at
 `481e2d4:docs/roadmap/implementation.md` in Git history.
@@ -216,6 +216,12 @@ Existing progress-cancellation and nonregular-file tests remain authoritative.
 
 ## Stage 5 — Establish Encoding Benchmarks And Error-Boundary Coverage
 
+**Status: Complete (2026-10-09).** `PrepareGeneration` now has an offline
+benchmark baseline for no extras, small extras, a 1 MiB string extra, nested
+JSON extras, and structured output. Tests protect nested invalid-extra context
+and request-error classification when the schema cannot be encoded. No
+production behavior changed.
+
 **Outcome:** A repeatable request-preparation baseline and focused protection
 for the classifications that Stage 6 must preserve. No production change.
 
@@ -744,6 +750,42 @@ through EOF. Focused verification passed: `go test -mod=readonly
 ./internal/artifact`; the existing cancellation and nonregular-file checks
 remain in the passing suite.
 
+### Request-Preparation Baseline
+
+The benchmark and boundary-test fixture is committed at `3f15715` (`Add
+request preparation baseline and boundary tests`). The benchmark measures
+`PrepareGeneration` without dispatch, with client construction, request
+fixtures, and the structured schema created before timing. It uses a synthetic
+endpoint and no credential or provider calls. Environment: Linux/amd64, Intel
+Core i5-7260U @ 2.20 GHz, benchmark parallelism 4, Go 1.26.8, ConfigKit
+`v0.5.0`, JSON Schema `v6.0.2`, and backend modules `v3.20260930.0`.
+
+Command (five 500 ms samples per case, without the race detector):
+
+```sh
+GOTOOLCHAIN=local GOWORK=off GOPROXY=off GONOPROXY=none GOSUMDB=off \
+  go test -mod=readonly -run '^$' -bench '^BenchmarkPrepareGeneration$' \
+  -benchmem -benchtime=500ms -count=5 ./internal/llm
+```
+
+Times list all five `ns/op` samples in command order, followed by median and
+range. Allocation figures are `B/op; allocs/op`.
+
+| Preparation case | `ns/op` samples (median; range) | Allocations |
+| --- | --- | --- |
+| No extras | 2771, 2873, 2872, 2907, 2885 (2873; 2771–2907) | 1632; 23 |
+| Small extras | 4216, 4255, 4440, 4090, 4245 (4245; 4090–4440) | 1928; 32 |
+| 1 MiB string extra | 3108655, 3046197, 3165484, 3115831, 3074278 (3108655; 3046197–3165484) | 2147898–2213942; 26–27 |
+| Nested JSON extras | 5933, 5803, 5675, 5490, 5611 (5675; 5490–5933) | 2304; 40 |
+| Structured output | 6421, 6018, 6479, 6153, 6712 (6421; 6018–6712) | 2632–2633; 47 |
+
+The baseline covers payload construction and complete JSON encoding for both
+ordinary and representative structured requests. The 1 MiB case's higher
+allocation volume is expected to include the large string and encoded body;
+these measurements are the comparison reference for the following request
+encoding change, not a provider-latency estimate. Focused verification passed:
+`go test -mod=readonly ./internal/llm`.
+
 ## Open Questions
 
 No consumer-policy decisions remain open. File reservation and encoding error