Use local builders for rendered templates

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 1–2 are complete; Stages 3–14 remain pending. Update status and evidence
+Stages 1–3 are complete; Stages 4–14 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.
@@ -150,6 +150,11 @@ family. Existing empty-output repair behavior remains unchanged.
 
 ## Stage 3 — Render Messages And Sessions With Local String Builders
 
+**Status: Complete (2026-10-09).** Message and session template execution now
+uses per-call `strings.Builder` values. Exact rendering, cancellation, and
+concurrent compiled-template behavior pass; benchmark evidence shows lower
+allocation without a measurable retained-output increase.
+
 **Outcome:** Template output becomes a string without a final buffer-to-string
 copy.
 
@@ -623,6 +628,61 @@ Focused verification passed on the implementation commit:
 `go test -mod=readonly ./internal/validate ./internal/usecase`. The benchmark
 family above was rerun on that commit with the recorded command.
 
+### Compiled-Rendering And Retained-Output Comparison
+
+The maintained Stage 1 benchmark fixture was measured before the rendering
+change at `dd89be3` (after the basic-validation change, which does not affect
+rendering), then again after replacing the message and session buffers in the
+working tree based on that revision. Setup and fixture contents are unchanged.
+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 for both runs (five 500 ms samples per benchmark, without the race
+detector):
+
+```sh
+GOTOOLCHAIN=local GOWORK=off GOPROXY=off GONOPROXY=none GOSUMDB=off \
+  go test -mod=readonly -run '^$' \
+  -bench '^(BenchmarkCompiledRendering|BenchmarkRetainedRenderedOutput)$' \
+  -benchmem -benchtime=500ms -count=5 ./internal/prompt
+```
+
+Times list all five `ns/op` samples in command order, followed by median and
+range. Allocation figures are `B/op; allocs/op`.
+
+| Compiled-render case | Before `ns/op` (median; range) | Before allocations | After `ns/op` (median; range) | After allocations |
+| --- | --- | --- | --- | --- |
+| Small message | 2066, 1906, 2003, 2027, 2010 (2010; 1906–2066) | 1632; 20 | 1912, 2030, 1877, 2045, 1926 (1926; 1877–2045) | 1568; 20 |
+| Artifact-backed 256 KiB message | 279116, 341635, 285472, 348345, 341764 (341635; 279116–348345) | 1050713–1050718; 25 | 165860, 239221, 255435, 223641, 214475 (223641; 165860–255435) | 788485–788489; 24 |
+| Repeated across session and messages | 966012, 1073914, 1098700, 1154416, 1064160 (1073914; 966012–1154416) | 3946015–3946038; 74 | 778584, 777091, 888988, 739473, 802395 (778584; 739473–888988) | 2446632–2446651; 71–72 |
+
+The retained-output workload holds four separately rendered 256 KiB results
+and reports post-GC live-heap change over the same setup-only fixture. Its
+allocation and retained-heap samples were:
+
+| Measurement | Before | After |
+| --- | --- | --- |
+| `ns/op` (median; range) | 331625, 333337, 343766, 302864, 329355 (331625; 302864–343766) | 244284, 239994, 238773, 239356, 264424 (239994; 238773–264424) |
+| `B/op; allocs/op` | 1050696–1050708; 25 | 788486–788495; 24 |
+| Retained heap (`retained-B`, median; range) | 1049744, 1046160, 1052768, 1044928, 1047264 (1047264; 1044928–1052768) | 1045824, 1054000, 1048288, 1047504, 1047168 (1047504; 1045824–1054000) |
+
+The artifact-backed case removes about 256 KiB of allocation per render, and
+the repeated case removes about 1.5 MiB; both also lose one allocation. Small
+rendering changes by 64 bytes and 0 allocations. The retained-heap ranges
+overlap, with medians differing by only 240 bytes for four 256 KiB outputs,
+so the returned strings keep approximately the same live output storage rather
+than retaining an additional builder buffer. This is expected: the builder's
+backing storage is the returned string instead of being copied into a second
+allocation. Measurements support retaining the simpler local-builder change
+without a material retention trade-off; the existing per-render artifact-text
+conversion and cache remain unchanged.
+
+Focused verification passed: `go test -race -mod=readonly ./internal/prompt`,
+including exact message/session output, cancellation, artifact-text lifecycle,
+and a new concurrent-render case that verifies independent request variables,
+artifact text, and session IDs against shared compiled templates.
+
 ## Open Questions
 
 No consumer-policy decisions remain open. File reservation and encoding error