Scan YAML framing preambles once

docs/roadmap/implementation.md

@@ -1,6 +1,6 @@
 # Performance implementation plan
 
-Status: implementation in progress; Stages 1–4 are complete, Stages 5–11 have not started.
+Status: implementation in progress; Stages 1–5 are complete, Stages 6–11 have not started.
 
 Date: 2026-10-09.
 
@@ -429,7 +429,7 @@ tree construction remains in each parse.
 
 ## Stage 5: scan YAML preambles without splitting documents
 
-**Status:** not started. **Prerequisite:** Stage 4.
+**Status:** complete. **Prerequisite:** Stage 4.
 
 **Files:** [show.go](../../show.go), [show_test.go](../../show_test.go),
 [configkit_test.go](../../configkit_test.go), framing benchmarks, and evidence.
@@ -464,7 +464,33 @@ tree construction remains in each parse.
 **Completion:** each document has one opening analysis, no document-wide line
 index, and the selected complete-buffer delivery model.
 
-**Evidence:** pending.
+**Evidence:** `GOWORK=off GOTOOLCHAIN=local GOPROXY=off GONOPROXY=none
+GOSUMDB=off go test -mod=readonly -count=1 .` passed. Existing aggregate tests
+continue to cover acquisition-before-output, directive document separators,
+decoded values, diagnostics, and writer failures. New framing cases cover a
+missing final newline, CRLF, a directive preamble, a control character in the
+scope label, unchanged native bytes, decoded values, and one-document output.
+
+Measurements used Go 1.26.8, linux/amd64, Linux 6.17.0-41-generic, Intel Core
+i5-7260U CPU @ 2.20 GHz, based on source revision `ae656d8` with this stage's
+changes in the working tree. The five-sample command passed:
+`GOWORK=off GOTOOLCHAIN=local GOPROXY=off GONOPROXY=none GOSUMDB=off go test -mod=readonly -run '^$' -bench '^BenchmarkPerformanceYAMLFraming$' -benchmem -benchtime=200ms -count=5 .`.
+Raw output is in `/tmp/commandkit-performance-yaml-after-stage5.txt`.
+
+Direct 4 KiB ordinary framing measured 1,135–1,455 ns/op, 4,864 B/op, and one
+allocation, compared with 5,883–6,842 ns/op, 11,056 B/op, and three allocations
+at baseline. Direct 1 MiB directive framing measured 189,303–359,655 ns/op,
+1,056,768–1,056,769 B/op, and one allocation, compared with 1,208,678–1,502,417
+ns/op, 2,539,520–2,539,523 B/op, and two allocations. Aggregate 4 KiB show
+measured 29,951–49,108 ns/op, 36,999–37,001 B/op, and 206 allocations, compared
+with 44,395–48,161 ns/op, 55,537–55,538 B/op, and 211 allocations. Aggregate
+1 MiB show measured 1,326,104–1,557,880 ns/op, 4,246,730–4,246,767 B/op, and
+211 allocations, compared with 4,818,907–6,009,877 ns/op, 8,695,182–8,695,197
+B/op, and 215–216 allocations. Single-scope 4 KiB display remained at
+17,935–17,936 B/op and 183 allocations; timing variation overlaps the baseline.
+The complete framed output still requires a full-size copy under OD-2. The
+allocation reduction comes from eliminating document-wide split/index storage
+and reusing the opening analysis for framing and aggregate directive handling.
 
 ## Stage 6: install validated registrations without copying the registry